Small interfering RNA mediated transcriptional gene silencing in mammalian cells

ABSTRACT

The present invention relates to transcriptional gene silencing (TGS) in mammalian, including human, cells that is mediated by small interfering RNA (siRNA) molecules. The present invention also relates to a method for directing histone and/or DNA methylation in mammalian, including human, cells. It has been found that siRNAs can be used to direct methylation of DNA in mammalian, including human, cells.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation-in-part of U.S. patentapplication Ser. No. 10/776,635 filed 12 Feb. 2004. U.S. patentapplication Ser. No. 10/776,635 is related to and claims priority under35 U.S.C. § 119(e) to U.S. provisional patent application Ser. No.60/447,013 filed 13 Feb. 2003. The present application is furtherrelated to and claims priority under 35 U.S.C. § 119(e) to U.S.provisional patent application Ser. No. 60/683,782 filed 24 May 2005.Each of these applications is incorporated herein by reference.

This application was made with Government support under Grant Nos.AI29329, AI42552, RO1 HL07470 and R01 HL83473 funded by the NationalInstitutes of Health Bethesda, Md. and under Grant No. 5P30 CA33572-21funded by National Cancer Institute, Bethesda, Md. The federalgovernment may have certain rights in this invention.

BACKGROUND OF THE INVENTION

The present invention relates to transcriptional gene silencing (TGS) inmammalian, including human, cells that is mediated by small interferingRNA (siRNA) molecules. The present invention also relates to a methodfor directing histone and/or DNA methylation in mammalian, includinghuman, cells. It has been found that siRNAs can be used to directmethylation of DNA in mammalian, including human, cells.

The publications and other materials used herein to illuminate thebackground of the invention, and in particular, cases to provideadditional details respecting the practice, are incorporated byreference, and for convenience are referenced in the following text byauthor and date and are listed alphabetically by author in the appendedbibliography.

RNA interference (RNAi) is a process in which double stranded RNA (dsRNA) induces the postranscriptional degradation of homologoustranscripts, and has been observed in a variety of organisms includingplants, fungi, insects, protozans, and mammals (Moss et al., 2001;Bernstein et al., 2001; Elbashir, et al., 2001a, 2001b). RNAi isinitiated by exposing cells to dsRNA either via transfection orendogenous expression. Double-stranded RNAs are processed into 21 to 23nucleotide (nt) fragments known as siRNA (small interfering RNAs)(Elbashir et al., 2001a, 2001b). These siRNAs form a complex known asthe RNA Induced Silencing Complex or RISC (Bernstein et al., 2001;Hammond et al. 2001), which functions in homologous target RNAdestruction. In mammalian systems, the sequence specific RNAi effect canbe observed by introduction of siRNAs either via transfection orendogenous expression of 21-23 base transcripts or longer hairpinprecursors. Use of siRNAs evades the dsRNA induced interferon and PKRpathways that lead to non-specific inhibition of gene expression.(Elbashir et al., 2001 a).

The discovery of siRNAs permitted RNAi to be used as an experimentaltool in higher eukaryotes. Typically, siRNAs are chemically synthesizedas 21mers with a central 19 bp duplex region and symmetric 2-base3′-overhangs on the termini. These duplexes are transfected into cellslines, directly mimicking the products made by Dicer in vivo. Most siRNAsequences can be administered to cultured cells or to animals withouteliciting an interferon response (Heidel et la., 2004; Ma et al., 2005;Judge et al., 2005). There are some reports that particular motifs caninduce such a response when delivered via lipids (Judge et al., 2005;Sledz et al., 2003; Homung eta l., 2005), although acyclodextrin-containing polycation system has been shown to deliversiRNA containing one such putative immunostimulatory motif that achievestarget gene down-regulation in mice without triggering an interferonresponse (Hu-Lieskovan et al., 2005), even in a disseminated tumormodel.

It has been recently described that chemically synthesized RNA duplexesof 25-30 base length can have as much as a 100-fold increase in potencycompared with 21mers at the same location. At the site most extensivelyexamined in this study, EGFPS1, only minor differences in potency wereseen between duplexes with blunt, 3′-overhang or 5′-overhang ends, and ablunt 27mer duplex was most potent (Kim et al., 2005). Increased potencyhas similarly been described for 29mer stem short hairpin RNAs (shRNAs)when compared with 19mer stem hairpins (Siolas et al., 2005). While theprimary function of Dicer is generally thought to be cleavage of longsubstrate dsRNAs into short siRNA products, Dicer also introduces thecleaved siRNA duplexes into nascent RISC in Drosophila (Lee et al.,2004); Pharm et al., 2004; Tomari et al., 2004). Dicer is involved inRISC assembly and is itself part of the pre-RISC complex (Sontheimer etal., 2005). The observed increased potency obtained using longer RNAs intriggering RNAi is theorized to result from providing Dicer with asubstrate (27mer) instead of a product (21mer) and that this improvesthe rate or efficiency of entry of the siRNA duplex into RISC.

Not all 27mers show this kind of increased potency. It is well knownthat shifting a 21mer siRNA by a few bases along the mRNA sequence canchange its potency by 10-fold or more (Holen et al., 2002); Harborth etal., 2003; Reynolds et al., 2004). Different products that result fromdicing can have different functional potency, and control of the dicingreaction may be necessary to best utilize Dicer-substrate RNAs in RNAi.The EGFPS1 blunt 27mer studied in Kim et al. (2005) is diced into twodistinct 21mers. Vermeulen and colleagues reported studies wheresynthetic 61mer duplex RNAs were digested using recombinant human Dicerin vitro and examined for cut sites using a ³²P-end-labeled gel assaysystem. Heterogeneous cleavage patterns were observed and the presenceof blunt versus 3′-overhang ends altered precise cleavage sites(Vermeulen et al., 2005). Dicing patterns were studied at a variety ofsites using different duplex designs to see if cleavage products couldbe predicted. It has been found that a wide variety of dicing patternscan result from blunt 27mer duplexes. An asymmetric duplex having asingle 2-base 3′-overhang generally has a more predictable and limiteddicing pattern where a major cleavage site is located 21-22 bases fromthe overhang. Including DNA residues at the 3′ end of the blunt side ofan asymmetric duplex further limits heterogeneity in dicing patterns andmakes it possible to design 27mer duplexes that result in predictableproducts after dicing.

It has been found that position of the 3′-overhang influences potencyand asymmetric duplexes having a 3′-overhang on the antisense strand aregenerally more potent than those with the 3′-overhang on the sensestrand (Rose et al., 2005). This can be attributed to asymmetricalstrand loading into RISC, as the opposite efficacy patterns are observedwhen targeting the antisense transcript. Novel designs described herethat incorporate a combination of asymmetric 3′-overhang with DNAresidues in the blunt end offer a reliable approach to designDicer-substrate RNA duplexes for use in RNAi applications. See also U.S.published application Nos. 2005/0244858 A1 and 2005/0277610 A1, eachincorporated herein by reference.

Recently, several groups have demonstrated that siRNAs can beeffectively transcribed by Pol III promoters in human cells and elicittarget specific mRNA degradation. (Lee et al., 2002; Miyagishi et al.,2002; Paul et al., 2002; Brummelkamp et al., 2002; Ketting et al.,2001). These siRNA encoding genes have been transiently transfected intohuman cells using plasmid or episomal viral backbones for delivery.Transient siRNA expression can be useful for rapid phenotypicdeterminations preliminary to making constructs designed to obtain longterm siRNA expression. Of particular interest is the fact that not allsites along a given mRNA are equally sensitive to siRNA mediateddownregulation. (Elbashir et al., 2001 a; Lee et al., 2001; Yu et al.,2002; Holen et al., 2002).

In contrast to post-transcriptional silencing involving degradation ofmRNA by short siRNAs, the use of long siRNAs to methylate DNA has beenshown to provide an alternate means of gene silencing in plants.(Hamilton et al., 2002). In higher order eukaryotes, DNA is methylatedat cytosines located 5′ to guanosine in the CpG dinucleotide. Thismodification has important regulatory effects on gene expression,especially when involving CpG-rich areas known as CpG islands, locatedin the promoter regions of many genes. While almost all gene-associatedislands are protected from methylation on autosomal chromosomes,extensive methylation of CpG islands has been associated withtranscriptional inactivation of selected imprinted genes and genes onthe inactive X-chromosomes of females. Aberrant methylation of normallyunmethylated CpG islands has been documented as a relatively frequentevent in immortalized and transformed cells and has been associated withtranscriptional inactivation of defined tumor suppressor genes in humancancers. In this last situation, promoter region hypermethylation standsas an alternative to coding region mutations in eliminating tumorsuppression gene function. (Herman et al., 1996).

U.S. published application No. 2004/0096843 A1, incorporated herein byreference, is directed to methods for producing double-stranded,interfering RNA molecules in mammalian cells. These methods overcomeprior limitations to the use of siRNA as a therapeutic agent invertebrate cells, including the need for short, highly defined RNAs tobe delivered to target cells other than through the use of synthetic,duplexed RNAs delivered exogenously to cells. U.S. published applicationNo. 2004/0091918 A1, incorporated herein by reference, is directed tomethods and kits for synthesis of siRNA expression kits.

Small interfering RNA (siRNA) mediated transcriptional gene silencing(TGS) was first observed in doubly transformed tobacco plants whichexhibited a suppressed phenotype of the transformed transgene. Carefulanalysis indicated that methylation of the targeted gene was involved inthe suppression(Matzke et al., 1989). TGS mediated by dsRNAs was furthersubstantiated in plants infected with a cytoplasmic dsRNA virus; nucleartransgenes with promoters homologous to sequences in the virus werefound to be silenced (Wassenegger et al., 1994; Wassenegger, 2000).siRNAs that target promoter sequences have also been shown to cause TGSin the yeast S. pombe and in Drosophila (Pal-Bhadra et al., 2002;Schramke and Allshire, 2003). Transcriptional silencing by siRNA mostlikely functions as a genome defense mechanisms that target chromatinmodifications to endogenous silent loci such as transposons and repeatedsequences (Seitz et al., 2003; Soifer et al., 2005). In plants and yeastsiRNA-induced silencing is accompanied by DNA methylation of homologoussequences, de novo DNA methylation in Arabidopsis thaliana requiressiRNA metabolizing factors, and maintenance of S. pombe centromericheterochromatin depends on siRNA-directed histone H3 lysine 9methylation (Chan et al., 2004; Jones et al., 2001; Mette et al., 2000;Volpe et al., 2002; Zilberman et al., 2003).

While dsRNAs induce sequence-specific methylation of DNA in plants andyeast, regulating gene expression at the transcriptional level, it wasnot known until recently how applicable this phenomenon was in mammaliancells. Recent reports have documented that siRNAs targeted to 2different genes, specifically the promoter regions, can inducetranscriptional silencing via histone and DNA methylation in human cells(Morris et al., 2004b; Kawasaki and Taira, 2004; Kawasaki et al., 2005).While these reports were intriguing many questions regarding theunderlying mechanism remained.

It is desired to utilize this activity and to use siRNAs to inducetranscriptional gene silencing in cells.

SUMMARY OF THE INVENTION

The present invention relates to transcriptional gene silencing (TGS) inmammalian, including human, cells that is mediated by small interferingRNA (siRNA) molecules. The present invention also relates to a methodfor directing histone and/or DNA methylation in mammalian, includinghuman, cells.

In one aspect of the invention, it has been found that siRNAs can beused to direct methylation of histones and/or DNA in mammalian,including human, cells.

In a second aspect of the invention, it has been found that theantisense strand from siRNA directed against a promoter sequence bindsDNMT3A to induce histone H3 lysine-27 methylation and transcriptionalgene silencing in an RNA polymerase II dependant manner.

In a third aspect of the invention, it has been found that Argonaute 1(Ago1) is required for siRNA mediated histone H3 lysine-9 di-methylation(H3K9^(me) ²⁺). It has also been found that Ago1 associates with RNApolymerase II (RNAPII), and that Ago1 and RNAPII co-localize toepigenetically silenced genomic loci, suggesting the involvement of anRNA component that is recognized by Ago1. Furthermore, the HIV-1 TARRNA-binding protein 2 (TRBP2) is enriched at silenced promoters, alongwith histone H3 lysine-27 tri-methylation (H3K27^(me3+)), a histonemethyl-mark that recruits the Polycomb group (PcG) repressor proteins.Thus, it has been found that Ago1 is involved in the initiation andspreading of siRNA mediated TGS, as well as transcriptional silencing atfacultative heterochromatin, linking the RNAi machinery with RNAPIItranscription and histone regulated control of gene expression.

In accordance with these findings, the present invention provides amethod of reducing gene expression of a target gene using an siRNAmolecule. In one embodiment, the siRNA molecule increases methylation ofhistones associated with the target gene. In a second embodiment, thesiRNA molecule is directed to the promoter region of the gene. In athird embodiment, the siRNA molecule binds to a sequence within about150 bp of the transcription start site.

In one aspect, the present invention provides a method for TGS in amammalian, including human, cell comprising exposing or introducing intothe cell a siRNA which is specific for a target sequence in the promoterregion of a gene to be silenced.

In another aspect, the present invention provides a method for TGS in amammalian, including human, cell comprising introducing into the cellDNA sequences encoding a sense strand and an antisense strand of ansiRNA which is specific for a target sequence in the promoter region ofa gene to be silenced, preferably under conditions permitting expressionof the siRNA in the cell, and wherein the siRNA induces histonemodifications characteristic of silent chromatin and/or methylation ofthe gene.

In a further aspect, the present invention provides a method for TGS ina mammalian, including human, cell comprising introducing into the cellan siRNA molecule which is specific for a target sequence in thepromoter region of a gene to be silenced and which interacts with Ago1to direct transcriptional silencing of the gene of interest.

In a still further aspect, the present invention provides siRNAmolecules, each comprising a sense strand and an antisense strand,wherein the antisense strand has a sequence sufficiently complementaryto a promoter region of a gene of interest to direct TGS of the gene ofinterest.

In another aspect, the present invention provides pharmaceuticalcompositions containing the disclosed siRNA molecules.

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1A-1D show a schematic representation of a polymerase chainreaction (PCR) strategy used to yield U6 transcription cassettesexpressing siRNAs. The 5′ PCR primer is complementary to the 5′ end ofthe U6 promoter and is standard for all PCR reactions. FIG. 1A: The 3′PCR primer is complementary to sequences at the 3′ end of the U6promoter and is followed by the sense or antisense sequences, a stretchof four to six deoxyadenosines (Ter) and an additional stuffer-Tagsequence. The adenosines are the termination signal for the U6 Pol IIIpromoter; therefore, any sequence added after this signal will not betranscribed by the Pol III polymerase and will not be part of the siRNA.FIG. 1B: The sense and antisense sequences are linked by a 9 nt loop andare inserted in the cassette by a two-step PCR reaction. FIG. 1C: Thesense and antisense sequences linked by a 9-nucleotide loop and followedby the stretch of adenosines and by the Tag sequences are included in asingle 3′ primer. FIG. 1D: Complete PCR expression cassette obtained bythe PCR reaction. To amplify and identify functional siRNAs from thetransfected cells, or to increase the yield of the PCR product shown inD, a nested PCR can be performed using the universal 5′ U6 primer and a3′ primer complementary to the Tag sequence, as indicated in the figure.

FIG. 2 shows the results of a methylation specific PCR (MSP) analysis ofthe RASSF1A promoter in siRNA transfected cells.

FIG. 3 illustrates DNA sequences of the RASSF1A promoter that becamemethylated in siRNA transfected cells.

FIG. 4 shows the results of RASSF1A intracellular expression in stableclones and cell populations (siPR28) transfected with specific shRNAs.

FIG. 5 shows the results of RASSF1A intracellular expression in stableclones transfected with 28 nucleotides shRNAs.

FIG. 6 shows the results of RNA down-regulation by shRNA directedagainst the RASSF1A promoter, as detected by transient transfections andquantitative PCR.

FIGS. 7A-7B show siRNA induced histone methylation. FIG. 7A: MPGtransfected EF52 siRNA induces histone methylation. Histone 3 Lysine 9(H3K9) di-methylation and histone 3 Lysine 27 (H3K27) tri-methylationwas determined from 293T cells transfected with EF52 or the control CCR5siRNAs (10 nM) using the nuclear specific amphipathic peptide MPG(Morris et al., 1997). Forty-eight hours post-transfection ChiP assayswere performed specifically for the EF1a promoter (Morris et al.,2004b). Results represent 2 independent experiments with standarddeviations shown. FIG. 7B: Nuclear specific delivery is required forHistone methylation. MPG and Lipofectamine trasnsfection reagents wereused to transfect EF52-Cy3+ siRNAs into 293T cells. Forty eight hoursfollowing transfection cultures were collected and a ChiP assay run.Results of a single experiment are shown.

FIGS. 8A-8C show siRNA pulldown assays and their results. FIG. 8A:Detection of flag-tagged DNMTs or HP1s bound to siRNA EF52. Schematicmethodology is shown for detecting biotin labeled siRNA which are boundby flag-tagged DNMTs. FIG. 8B: Co-immunoprecipitation ChiP/siRNA assay.Methodology for performing a H3K9 or H3K27 ChiP followed by abiotin/avidin pulldown for detecting siRNA EF52 associated with histonemethyl marks (H3K9 or H3K27). FIG. 8C: Triple immunoprecipitation assay.Methodology is shown for performing first a ChiP for H3K27 followed by aflag-immunoprecipitation followed by a biotin/avidin pulldown for biotinlabeled siRNA. The resultant elutes were utilized in PCR for thetargeted EF1A promoter.

FIGS. 9A-9E show the expression of the flagged proteins. FIG. 9A Biotinlabeled EF52 siRNAs pulldown DNMT3A. Whole cell lysates from transfected293T cells expressed detectable amounts of all flag-tagged expressedDNMT proteins (Table 1) with the exception of DNMT 3B1. FIG. 9B:Recombinant HP1 alpha and beta were also expressed at appreciable levelsin whole cell lysates while HP1-gamma exhibiting a reduced expression.FIG. 9C: Following siRNA EF52-Biotin incubation with whole cell extracts(A&B) and subsequent pull-down (FIG. 8A) only DNMT 3A, 3A2 and 3B2 weredetectable while the control Mock, Prp2, and DNMT-1 (MT-1) were notco-immunoprecipitated with the EF52 siRNA. FIG. 9D: Flag-tagged DNMT1and DNMT3A transfected 293T lysates (whole cell extracts, refer to FIG.8A) were generated and incubated with a total of 500 nM siRNA biotinlabeled Sense (S), antisense (AS), or both sense and antisense (S/AS),and the control sense and antisense without a biotin label (−). FIG. 9E:Antisense and the Sense/Antisense siRNA EF52 binds DNMT3A but not DNMT1.

FIG. 10 shows detection of the antisense strand in flag-tag pulldowns.Flag-tagged DNMT3A and control (mock lysates alone) were incubated with500 nM of siRNA EF52 and bound siRNA detected by binding of theradiolabelled probe to the respective target strand, sense or antisense.Results are representative of a single experiment.

FIG. 11A-11E show analysis of siRNA and H3K27. FIG. 11A: Detection ofantisense siRNA/H3K27 and targeted EF1A promoter as one complex. Biotinlabeled antisense siRNA EF52 co-precipitates (˜4.8 fold greaterconcentration relative to no antibody control) with tri-methylated H3K27in a ChiP/RNA co-immunoprecipitation. FIG. 11B: Biotin labeled antisensesiRNA EF52 co-precipitates with H3K27, flag-tagged DNMT3A and thetargeted EF1A promoter. FIG. 11C: HIV-1 U3 specific antisense siRNAsLTR-247 and LTR-362 suppress Tat induced luciferase expression in TZM-B1 cells. Results from a single experiment are shown. FIG. 11D: AntisensesiRNAs targeting the U3 region of the HIV-1 LTR inhibit Tat mediatedactivation of fire fly luciferase in 1G5 cells. Results are from twoindependent experiments and standard deviations are shown. FIG. 11E:Treatment of 293T cells with alpha amanatin (0.05 μg/ml) 24 hrsfollowing transfection with siRNA EF52 reduces H3K9 methylation ˜60%relative to no antibody control to levels comparable to CCR5 siRNAtransfected cultures (not shown). Results from one experiment are shown.

FIG. 12 shows LTR specific siRNAs induce silencing of Tat mediatedexpression of fire fly luciferase. HIV-1 U3 LTR specific siRNAs (Table2) targeting either subtype B or subtype c were co-transfected withpCMV-Tat expression plasmid into TZM-B1 (Wei et al., 2002) cells andluciferase expression determined 48 hrs later. Results from twoindependent experiments are shown with standard deviations.

FIG. 13 shows a model for siRNA mediated TGS in human cells on the basisof the results up to this figure. SiRNAs are introduced by nuclearspecific MPG based transfection (Morris et al., 2004b) into the targetcells (1). Once inside the nucleus the antisense strand of the siRNA(AS-siRNA) is bound by DNMT3A (data not shown) (2). (DNMT3b may alsobind siRNA)(Jeffery and Nakielny, 2004; K. V. data not shown). Next theAS-siRNA/DNMT3a complex may interact directly or already be bound byHDACs and/or Suv39H1 (Fuks et al., 2003; Fuks et al. 2001) (3). TheAS-siRNA probably then directs either the AS-siRNA/DNMT3A complex withor without the HDACs and/or Suv39H1 to the targeted promoter region,possibly via an interaction with a non-coding transcript that isassociated with the targeted chromatin (4) where HDAC can deacetylatethe respective histones (H3K9 and/or H3K27). The deacetylation of H3K9and H3K27 would then permit histone methyltransferases such as Suv39H1to methylate H3K9 and possibly H3K27 resulting in initial silencing oftranscription (5). If the silencing is re-enforced the gene may becomemethylated and permanently silenced.

FIG. 14 shows CCR5 promoter-targeted knockdown of GFP expression.Suppression of CCR5 expressed GFP by promoter-specific siRNAs at 48 hrspost-transfection. Error bars represent standard deviations from n=3independent experiments.

FIGS. 15A-15C show that synthetic siRNAs and expressed shRNAs mediatetranscriptional gene silencing of the CCR5 and RASSF1A promoters. FIG.15A: GFP mRNA expression in R61 (promoter-specific) or R5 (CCR5mRNA-specific) control siRNA-treated 293T CCR5-GFP cells, and RASSF1AmRNA expression in HeLa cells stably expressing an shRNA (RASSF1Apromoter-specific) or control vector alone, as determined by real-timequantitative RT-PCR (qRT-PCR) and normalized to GAPDH levels (at 24 hrspost-siRNA transfection for GFP samples). Error bars represent standarderror of the mean (s.e.m.) for n=4 (GFP) and n=3 (RASSF1A) independentsamples, respectively. FIG. 15B: Chromatin immunoprecipitation (ChIP) ofthe CCR5 promoter (at the R61 siRNA target site or 100-300 bpdownstream) using anti-H3K9^(me2+) antibody in extracts from R61 or R5control siRNA-treated cells at 24 hrs post-siRNA transfection. Errorbars represent s.e.m. for n=3 independent experiments. FIG. 15C:Time-course ChIP of the CCR5 promoter using anti-H3K9^(me2+) antibody inextracts from R61 or R5 control siRNA-treated cells at 12 and 24 hrspost-siRNA transfection.

FIG. 16 shows low levels of DNA methylation at targeted promoters. DNAmethylation at the R61 or R5 control siRNA-targeted CCR5 promoter in293T CCR5-GFP cells, and DNA methylation at the endogenous RASSF1Apromoter in HeLa stable cells expressing promoter-specific shRNA orcontrol vector, using an AvaI or ApaI-based DNA methylation assay of theCCR5 and RASSF1A promoters, respectively. Error bars represent standarderror of the mean (s.e.m.) for n=3 independent samples.

FIGS. 17A-17C show that Argonaute 1 protein associates with the targetedCCR5 and RASSF1A promoters and RNA polymerase II. FIG. 17A: ChIP of theCCR5 promoter (at the R61 siRNA target site or 100-300 bp downstream) inextracts from 293T CCR5-GFP cells transfected with R61 or R5 controlsiRNAs at 18 hrs post-siRNA transfection, and ChIP of the endogenousRASSF1A promoter in extracts from promoter shRNA-expressing or controlvector-expressing HeLa stable cells, using anti-Ago1 antibody. Errorbars represent s.e.m. for n=3 independent experiments. FIG. 17B:Time-course ChIP of the CCR5 promoter performed at 6 hr intervalspost-R61 or R5 control siRNA transfection. FIG. 17C: Whole cell extracts(WCE) from 293T CCR5-GFP cells and anti-RNAPII immunoprecipitates fromRNase A untreated (−RNase A) or treated (+RNase A) extracts, analyzed bywestern blot using anti-Ago1 antibody.

FIGS. 18A-18B Argonaute 2 does not associate with the siRNA targetedCCR5 promoter. ChIP was performed using anti-Ago2 antibody on theCCR5-GFP promoter in 293T CCR5-GFP cells transfected with either R61 orR5 control siRNAs at 18 hrs post-siRNA transfection (FIG. 18A) or on theendogenously silenced CCR5 promoter in HEK 293 cells using anti-Ago2antibody or no antibody controls (FIG. 18B). Error bars represent s.e.m.for n=3 independent experiments.

FIGS. 19A-19C show that Argonaute 1 is required for histone methylationand transcriptional silencing. FIG. 19A: Whole cell extracts (WCE) andextracts from 293T CCR5-GFP cells treated with Ago1 mRNA-specific siRNA[Ago1(−)] at 48 hrs post-Ago1 siRNA transfection and analyzed by westernblotting using anti-Ago1 antibody. GFP was included as a loadingcontrol. FIG. 19B: 293T CCR5-GFP cells transfected with R61 siRNA at 24hrs post-R5 control siRNA [Ago1(+)] or Ago1 siRNA [Ago1(−)]transfection, as determined by qRT-PCR and normalized to GAPDH levels at24 hrs post-R61 siRNA transfection. Error bars represent s.e.m. for n=3independent samples. FIG. 19C: ChIP of the CCR5 promoter using anti-Ago1or anti-H3K9^(me2+) antibody in R61 or R5 control siRNA-treated Ago1(−)293T CCR5-GFP cells at 24 hrs post-R61 or R5 control siRNA transfectionand 48 hrs post-Ago1 siRNA transfection. Error bars represent s.e.m. forn=3 independent experiments.

FIGS. 20A-20C show that Argonaute 1, RNA polymerase II, H3K27^(me3+),and TRBP are enriched at the endogenously silenced CCR5 promoter. ChIPof the endogenous CCR5 promoter in untreated HEK 293 (FIG. 20A) and HeLa(FIG. 20B) cell extracts, using anti-Ago1, anti-RNAPII, andanti-H3K27^(me3+) antibodies or no antibody controls and normalized toinput values. Error bars represent s.e.m. for n=3 independentexperiments. FIG. 20C: ChiP of the endogenous RASSF1A promoter inextracts from promoter shRNA-expressing or control vector-expressingHeLa stable cells, and ChIP of the endogenous CCR5 promoter in HeLa cellextracts, using anti-TRBP antiserum. Error bars represent s.e.m. for n=3independent experiments.

FIG. 21 shows a model for the mechanism of Ago1 directed TGS. Thetranscriptional silencing complex (TSC) may contain Ago1, TRBP, siRNA,and histone methyltransferases EZH2 (H3K27^(me3+)) and G9a(H3K9^(me2)+). The tri-methlation of H3K would subsequently allow forthe Polycomb group repressor complexes to bind to H3K27^(me3+) andrecruit DNMT3a, locking in an epigenetically silent state. DNMT3a hasalso been shown to co-immunoprecipitate with both the antisense strandof the siRNA and the H₃K27^(me3+) methyl-mark (Weinberg et al., 2006).

FIG. 22 shows that Polycomb group protein EZH2 is enriched at theRASSF1A and CCR5 promoters. ChIP of the endogenously expressed RASSF1Apromoter in extracts from promoter shRNA-expressing or controlvector-expressing HeLa stable cells, and ChIP of the endogenouslysilenced CCR5 promoter in HeLa cell extracts, using anti-EZH2 antibody.Error bars represent s.e.m. for n=3 independent experiments.

FIG. 23 shows that Argonaute 1 is enriched at the Polycomb group targetMYT1 promoter. ChIP of the endogenous Polycomb group target MYT1promoter in untreated HeLa extracts, using anti-Ago1, anti-EZH2, andanti-H3K27^(me3+) antibodies or no antibody controls and normalized toinput values. Error bars represent s.e.m. for n=3 independentexperiments.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to transcriptional gene silencing (TGS) inmammalian, including human, cells that is mediated by small interferingRNA (siRNA) molecules. The present invention also relates to a methodfor directing histone and/or DNA methylation in mammalian, includinghuman, cells.

Small interfering RNAs (siRNAs) silence genes at the transcriptional andpost-transcriptional level in human cells. As shown herein siRNAs areable to direct transcription gene silencing via a number of mechanismsincluding, but not limited to, the methylation of human genes and/orassociated histones in both the promoter and coding regions of the gene.In addition, although siRNA mediated transcriptional gene silencing(TGS) was recently reported (Morris et al., 2004b) the mechanismremained relatively unknown. As shown herein, we have expanded on thisinitial observation to address the mechanism of siRNA mediated TGS byscreening the binding potential of DNA methyltransferases (DNMT) 1, 3A,3A2, 3B1, 3B2, and heterochromatin proteins (HP1-alpha, beta, and gamma)to the promoter targeted EF52 siRNAs. Interestingly, DNMT3A, 3A2, 3B1and 3B2 bound EF52 with DNMT3A displaying the most robust binding.DNMT3A co-immunoprecipitated with the antisense strand of the EF52siRNA, the targeted EF1 alpha promoter, and the corresponding silentstate histone methyl mark. Moreover, the induction of a silent statehistone methylation mark by the EF52 siRNA was contingent on RNApolymerase II (Pol-II). The functionality of the antisense strand toinduce TGS was also confirmed by targeting the U3 region of thepromoter/LTR of HIV-1. These data implicate a functional link betweensiRNA mediated targeting of genomic regions (including promoters), DNAmethylation and DNA methyltransferases (DNMTs), and chromatin remodelingcomplexes (Suv39H1 and HDACs) in human cells. Moreover, the observationssuggest that the antisense strand can induce TGS, this interaction isvia an antisense/DNA interaction and requires Pol-II to putatively openup the targeted promoter and as such presents a completely newmethodology to transcriptionally silence Pol-II promoter expressedgenes.

In one aspect of the invention, design elements to promote TGS are shownherein for the siRNA EF52 to the EF1A promoter which was previouslyshown to induce transcriptional silence of the EF1A promoter (Morris etal., 2004b). Previous studies have demonstrated that transcriptionalinhibition was associated with de novo DNA methylation within thesiRNA-targeted sequence, and was relieved with the drugs 5′azacytidine(5′-AzaC) and trichostatin A (TSA), inhibitors of DNA methylation andhistone deacetylation respectively. Notably, gene silencing required thesiRNAs access to the nucleus in order to down-regulate transcription. Wedemonstrate here that siRNA induced histone methylation (Histone 3Lysine 9 and Histone 3 Lysine 27, H3K9 and H3K27, respectively) of thetargeted promoter is dependent on nuclear specific delivery of the EF52siRNA, is the result of the antisense strand, and requires RNApolymerase II. Moreover, we show direct evidence that siRNA EF52 bindsDNMT3A in a strand specific manner and co-immunoprecipitated not onlywith the flag-tagged DNMT3A but also the targeted promoter in a H3K27methyl-specific manner. The observations of strand specific siRNAmediated TGS was substantiated by targeting the U3 region of the HIV-1LTR/promoter.

In a second aspect of the invention, we describe the role of Argonaute 1(Ago1) in directing transcriptional silencing at both the chemokinereceptor CCR5 (HIV-1 co-receptor) and the tumor suppressor RASSF1Apromoters. Ago1 is required for siRNA mediated histone H3 lysine-9di-methylation (H3K9^(me2+)) at the targeted CCR5 promoter, andknockdown of Ago1 results in the loss of H3K9^(me2+), disrupting theoverall potency of TGS. Co-immunoprecipitations indicate that Ago1associates with RNA polymerase II (RNAPII), and chromatinimmunoprecipitations (ChiP) of endogenously silenced CCR5 promoters showthat Ago1 and RNAPII co-localize to epigenetically silenced genomicloci, suggesting the involvement of an RNA component that is recognizedby Ago1. Furthermore, the HIV-1 TAR RNA-binding protein 2 (TRBP2) isenriched at silenced promoters, along with histone H3 lysine-27tri-methylation (H3K27^(me3+)) , a histone methyl-mark that recruits thePolycomb group (PcG) repressor proteins. Our results suggest that Ago1is involved in the initiation and spreading of siRNA mediated TGS, aswell as transcriptional silencing at facultative heterochromatin,linking the RNAi machinery with RNAPII transcription and histoneregulated control of gene expression.

In a third aspect of the invention, a model for siRNA mediated TGS inhuman cells involving a transcriptional silencing complex (TSC)containing Ago1, TRBP2, siRNA, and possibly chromatin remodeling factors(i.e. HDAC-1, G9a, EZH2, DNMT3a). The TSC may be directed by siRNAs totheir target promoters in an RNAPII-dependent manner, and theobservation here that Ago1 associates with RNAPII suggests that RNAPIImay provide a docking site for the TSC. Upon siRNA loading into the TSC,the antisense strand may guide the TSC to a low copy promoter-specificRNA (pRNA) that corresponds to the siRNA targeted promoter. This wouldallow for the formation of an RNA:RNA duplex between the antisensestrand of the siRNA and either a nascent pRNA while it is beingtranscribed or a pRNA that is already a component of the local chromatinstructure. Recognition of the siRNA target site would potentially stallthe pRNA-scanning TSC:RNAPII complex and initiate the formation offacultative heterochromatin by recruiting histone methyltransferases andpossibly PcG repressor complexes, which have recently been linked toAgo1 and the RNAi machinery in Drosophila. The inclusion of TRBP2 in theTSC suggests a potentially important role for this protein in Ago1mediated RNA binding.

An alternative model implicated by the observed spreading of TGS andfacultative heterochromatin from a promoter nucleation site wouldinvolve the siRNA antisense strand-directed TSC:RNAPII complex movingalong the targeted RNAPII-transcribed promoter/gene, potentiallymodifying the H3 histones as they are reconstituted into nucleosomesimmediately following transcription. Both of these models, or anamalgamation of the two, would necessitate the involvement of RNAPII,which is consistent with recent evidence that RNAPII function isrequired for histone methylation and TGS at siRNA-targeted promoters inhuman cells and in S. Pombe, suggesting an Ago1 and RNAPII-dependentmechanism of transcriptional silencing that is evolutionarily conserved.Additionally, the recent discovery and characterization of a vast arrayof small (21- to 26-nt), non-coding RNAs is changing the classicalunderstanding of gene regulation, and taken together with the datapresented here, suggests that these non-coding RNAs may play a moreprofound role in writing the histone code and regulating gene expressionat the level of DNA.

Thus, the present invention provides a method of reducing geneexpression of a target gene using an siRNA molecule. In one embodiment,the siRNA molecule increases methylation of histones associated with thetarget gene. In a second embodiment, the siRNA molecule is directed tothe promoter region of the gene. In a third embodiment, the siRNAmolecule binds to a sequence within about 150 bp of the transcriptionstart site. As used herein, the term transcriptional start site refersto the nucleotide in a gene from which transcription is initiated andlies between the TATA box (TATA or TATAA sequences) and the translationinitiation site.

In one aspect, the present invention provides a method for TGS in amammalian, including human, cell comprising exposing or introducing intothe cell a siRNA which is specific for a target sequence in the promoterregion of a gene to be silenced.

In another aspect, the present invention provides a method for TGS in amammalian, including human, cell comprising introducing into the cellDNA sequences encoding a sense strand and an antisense strand of ansiRNA which is specific for a target sequence in the promoter region ofa gene to be silenced, preferably under conditions permitting expressionof the siRNA in the cell, and wherein the siRNA induces histonemodifications characteristic of silent chromatin and/or methylation ofthe gene.

In a further aspect, the present invention provides a method for TGS ina mammalian, including human, cell comprising introducing into the cellan siRNA molecule which is specific for a target sequence in thepromoter region of a gene to be silenced and which interacts with Ago1to direct transcriptional silencing of the gene of interest.

In a still further aspect, the present invention provides siRNAmolecules, each comprising a sense strand and an antisense strand,wherein the antisense strand has a sequence sufficiently complementaryto a promoter region of a gene of interest to direct TGS of the gene ofinterest.

In another aspect, the present invention provides pharmaceuticalcompositions containing the disclosed siRNA molecules.

Possible target genes for TGS in a mammalian, including human, cellinclude those associated with disease, including those involved withresponse to infectious agents (e.g., bacteria, viruses, fungi, etc.),cancer genes, genes leading to disease, or any gene for which TGS isdesired.

The siRNA molecule may have different forms, including a single strand,a paired double strand (dsRNA) or a hairpin (shRNA) and can be produced,for example, either sythetically or by expression in cells. In oneembodiment, DNA sequences for encoding the sense and antisense strandsof the siRNA molecule to be expressed directly in mammalian cells can beproduced by methods known in the art, including but not limited to,methods described in U.S. published application Nos. 2004/0171118 A1,2005/0244858 A1 and 2005/0277610 A1, each incorporated herein byreference.

In one aspect of the invention, DNA sequences encoding a sense strandand an antisense strand of a siRNA specific for a target sequence of agene are introduced into mammalian cells for expression. To target morethan one sequence in the gene (such as different promoter regionsequences and/or coding region sequences), separate siRNA-encoding DNAsequences specific to each targeted gene sequence can be introducedsimultaneously into the cell. In accordance with another embodiment,mammalian cells may be exposed to multiple siRNAs that target multiplesequences in the gene.

The siRNA molecules generally contain about 19 to about 30 base pairs,and preferably are designed to cause methylation of the targeted genesequence. In one embodiment, the siRNA molecules contain about 19-23base pairs, and preferably about 21 base pairs. In another embodiment,the siRNA molecules contain about 24-28 base pairs, and preferably about26 base pairs. In a further embodiment, the dsRNA has an asymmetricstructure, with the sense strand having a 25-base pair length, and theantisense strand having a 27-base pair length with a 2 base 3′-overhang.In another embodiment, this dsRNA having an asymmetric structure furthercontains 2 deoxynucleotides at the 3′end of the sense strand in place oftwo of the ribonucleotides. Individual siRNA molecules also may be inthe form of single strands, as well as paired double strands (“sense”and “antisense”) and may include secondary structure such as a hairpinloop. Individual siRNA molecules could also be delivered as precursormolecules, which are subsequently altered to give rise to activemolecules. Examples of siRNA molecules in the form of single strandsinclude a single stranded anti-sense siRNA against a non-transcribedregion of a DNA sequence (e.g. a promoter region).

The sense and antisense strands anneal under biological conditions, suchas the conditions found in the cytoplasm of a cell. In addition, aregion of one of the sequences, particularly of the antisense strand, ofthe dsRNA has a sequence length of at least 19 nucleotides, whereinthese nucleotides are adjacent to the 3′end of antisense strand and aresufficiently complementary to a nucleotide sequence of the RNA producedfrom the target gene.

The precursor RNAi molecule, may also have one or more of the followingadditional properties: (a) the antisense strand has a right shift fromthe typical 21mer and (b) the strands may not be completelycomplementary, i.e., the strands may contain simple mismatch pairings. A“typical” 21mer siRNA is designed using conventional techniques, such asdescribed above. This 21mer is then used to design a right shift toinclude 1-7 additional nucleotides on the 5′ end of the 21mer. Thesequence of these additional nucleotides may have any sequence. Althoughthe added ribonucleotides may be complementary to the target genesequence, full complementarity between the target sequence and the siRNAis not required. That is, the resultant siRNA is sufficientlycomplementary with the target sequence. The first and secondoligonucleotides are not required to be completely complementary. Theyonly need to be substantially complementary to anneal under biologicalconditions and to provide a substrate for Dicer that produces a siRNAsufficiently complementary to the target sequence. In one embodiment,the dsRNA has an asymmetric structure, with the antisense strand havinga 25-base pair length, and the sense strand having a 27-base pair lengthwith a 2 base 3′-overhang. In another embodiment, this dsRNA having anasymmetric structure further contains 2 deoxynucleotides at the 3′end ofthe antisense strand.

Suitable dsRNA compositions that contain two separate oligonucleotidescan be linked by a third structure. The third structure will not blockDicer activity on the dsRNA and will not interfere with the directeddestruction of the RNA transcribed from the target gene. In oneembodiment, the third structure may be a chemical linking group. Manysuitable chemical linking groups are known in the art and can be used.Alternatively, the third structure may be an oligonucleotide that linksthe two oligonucleotides of the dsRNA is a manner such that a hairpinstructure is produced upon annealing of the two oligonucleotides makingup the dsRNA composition. The hairpin structure will not block Diceractivity on the dsRNA and will not interfere with the directeddestruction of the RNA transcribed from the target gene.

The sense and antisense sequences may be attached by a loop sequence.The loop sequence may comprise any sequence or length that allowsexpression of a functional siRNA expression cassette in accordance withthe invention. In a preferred embodiment, the loop sequence containshigher amounts of uridines and guanines than other nucleotide bases. Thepreferred length of the loop sequence is about 4 to about 9 nucleotidebases, and most preferably about 8 or 9 nucleotide bases.

In another embodiment of the present invention, the dsRNA, i.e., theprecursor RNAi molecule, has several properties which enhances itsprocessing by Dicer. According to this embodiment, the dsRNA has alength sufficient such that it is processed by Dicer to produce an siRNAand at least one of the following properties: (i) the dsRNA isasymmetric, e.g., has a 3′ overhang on the sense strand and (ii) thedsRNA has a modified 3′ end on the antisense strand to directorientation of Dicer binding and processing of the dsRNA to an activesiRNA. According to this embodiment, the longest strand in the dsRNAcomprises 24-30 nucleotides. In one embodiment, the sense strandcomprises 24-30 nucleotides and the antisense strand comprises 22-28nucleotides. Thus, the resulting dsRNA has an overhang on the 3′ end ofthe sense strand. The overhang is 1-3 nucleotides, such as 2nucleotides. The antisense strand may also have a 5′ phosphate.

Modifications can be included in the dsRNA, i.e., the precursor RNAimolecule, so long as the modification does not prevent the dsRNAcomposition from serving as a substrate for Dicer. In one embodiment,one or more modifications are made that enhance Dicer processing of thedsRNA. In a second embodiment, one or more modifications are made thatresult in more effective RNAi generation. In a third embodiment, one ormore modifications are made that support a greater RNAi effect. In afourth embodiment, one or more modifications are made that result ingreater potency per each dsRNA molecule to be delivered to the cell.Modifications can be incorporated in the 3′-terminal region, the5′-terminal region, in both the 3′-terminal and 5′-terminal region or insome instances in various positions within the sequence. With therestrictions noted above in mind any number and combination ofmodifications can be incorporated into the dsRNA. Where multiplemodifications are present, they may be the same or different.Modifications to bases, sugar moieties, the phosphate backbone, andtheir combinations are contemplated. Either 5′-terminus can bephosphorylated.

In another embodiment, the antisense strand is modified for Dicerprocessing by suitable modifiers located at the 3′ end of the antisensestrand, i.e., the dsRNA is designed to direct orientation of Dicerbinding and processing. Suitable modifiers include nucleotides such asdeoxyribonucleotides, dideoxyribonucleotides, acyclonucleotides and thelike and sterically hindered molecules, such as fluorescent moleculesand the like. Acyclonucleotides substitute a 2-hydroxyethoxymethyl groupfor the 2′-deoxyribofuranosyl sugar normally present in dNMPs. Othernucleotide modifiers could include 3′-deoxyadenosine (cordycepin),3′-azido-3′-deoxythymidine (AZT), 2′,3′-dideoxyinosine (ddI),2′,3′-dideoxy-3′-thiacytidine (3TC),2′,3′-didehydro-2′,3′-dideoxythymidine (d4T) and the monophosphatenucleotides of 3′-azido-3′-deoxythymidine (AZT),2′,3′-dideoxy-3′-thiacytidine (3TC) and2′,3′-didehydro-2′,3′-dideoxythymidine (d4T). In one embodiment,deoxynucleotides are used as the modifiers. When nucleotide modifiersare utilized, 1-3 nucleotide modifiers, or 2 nucleotide modifiers aresubstituted for the ribonucleotides on the 3′ end of the antisensestrand. When sterically hindered molecules are utilized, they areattached to the ribonucleotide at the 3′ end of the antisense strand.Thus, the length of the strand does not change with the incorporation ofthe modifiers. In another embodiment, the invention contemplatessubstituting two DNA bases in the dsRNA to direct the orientation ofDicer processing. In a further invention, two terminal DNA bases arelocated on the 3′ end of the antisense strand in place of tworibonucleotides forming a blunt end of the duplex on the 5′ end of thesense strand and the 3′ end of the antisense strand, and atwo-nucleotide RNA overhang is located on the 3′-end of the sensestrand. This is an asymmetric composition with DNA on the blunt end andRNA bases on the overhanging end.

Examples of modifications contemplated for the phosphate backboneinclude phosphonates, including methylphosphonate, phosphorothioate, andphosphotriester modifications such as alkylphosphotriesters, and thelike. Examples of modifications contemplated for the sugar moietyinclude 2′-alkyl pyrimidine, such as 2′-O-methyl, 2′-fluoro, amino, anddeoxy modifications and the like (see, e.g., Amarzguioui et al., 2003).Examples of modifications contemplated for the base groups includeabasic sugars, 2-O-alkyl modified pyrimidines, 4-thiouracil,5-bromouracil, 5-iodouracil, and 5-(3-aminoallyl)-uracil and the like.Locked nucleic acids, or LNA's, could also be incorporated. Many othermodifications are known and can be used so long as the above criteriaare satisfied. Examples of modifications are also disclosed in U.S. Pat.Nos. 5,684,143, 5,858,988 and 6,291,438 and in U.S. published patentapplication No. 2004/0203145 A1, each incorporated herein by reference.Other modifications are disclosed in Herdewijn (2000), Eckstein (2000),Rusckowski et al. (2000), Stein et al. (2001) and Vorobjev et al.(2001), each incorporated herein by reference.

Additionally, the siRNA structure can be optimized to ensure that theoligonucleotide segment generated from Dicer's cleavage will be theportion of the oligonucleotide that is most effective in inhibiting geneexpression. For example, in one embodiment of the invention a 27-bpoligonucleotide of the dsRNA structure is synthesized wherein theanticipated 21 to 22-bp segment that will inhibit gene expression islocated on the 3′-end of the antisense strand. The remaining baseslocated on the 5′-end of the antisense strand will be cleaved by Dicerand will be discarded. This cleaved portion can be homologous (i.e.,based on the sequence of the target sequence) or non-homologous andadded to extend the nucleic acid strand.

RNA may be produced enzymatically or by partial/total organic synthesis,and modified ribonucleotides can be introduced by in vitro enzymatic ororganic synthesis. In one embodiment, each strand is preparedchemically. Methods of synthesizing RNA molecules are known in the art,in particular, the chemical synthesis methods as described in Verma andEckstein (1998) or as described herein.

In another aspect, the present invention provides for a pharmaceuticalcomposition comprising the siRNA of the present invention. The siRNAsample can be suitably formulated and introduced into the environment ofthe cell by any means that allows for a sufficient portion of the sampleto enter the cell to induce gene silencing, if it is to occur. Manyformulations for dsRNA are known in the art and can be used so long assiRNA gains entry to the target cells so that it can act. See, e.g.,U.S. published patent application Nos. 2004/0203145 A1 and 2005/0054598A1, each incorporated herein by reference. For example, siRNA can beformulated in buffer solutions such as phosphate buffered salinesolutions, liposomes, micellar structures, and capsids. Formulations ofsiRNA with cationic lipids can be used to facilitate transfection of thedsRNA into cells. For example, cationic lipids, such as lipofectin (U.S.Pat. No. 5,705,188, incorporated herein by reference), cationic glycerolderivatives, and polycationic molecules, such as polylysine (publishedPCT International Application WO 97/30731, incorporated herein byreference), can be used. Suitable lipids include Oligofectamine,Lipofectamine (Life Technologies), NC388 (Ribozyme Pharmaceuticals,Inc., Boulder, Colo.), or FuGene 6 (Roche) all of which can be usedaccording to the manufacturer's instructions.

It can be appreciated that the method of introducing siRNA into theenvironment of the cell will depend on the type of cell and the make upof its environment. For example, when the cells are found within aliquid, one preferable formulation is with a lipid formulation such asin lipofectamine and the siRNA can be added directly to the liquidenvironment of the cells. Lipid formulations can also be administered toanimals such as by intravenous, intramuscular, or intraperitonealinjection, or orally or by inhalation or other methods as are known inthe art. When the formulation is suitable for administration intoanimals such as mammals and more specifically humans, the formulation isalso pharmaceutically acceptable. Pharmaceutically acceptableformulations for administering oligonucleotides are known and can beused. In some instances, it may be preferable to formulate siRNA in abuffer or saline solution and directly inject the formulated dsRNA intocells, as in studies with oocytes. The direct injection of dsRNAduplexes may also be done. For suitable methods of introducing siRNA seeU.S. published patent application No. 2004/0203145 A1, incorporatedherein by reference.

Suitable amounts of siRNA must be introduced and these amounts can beempirically determined using standard methods. Typically, effectiveconcentrations of individual siRNA species in the environment of a cellwill be about 50 nanomolar or less 10 nanomolar or less, or compositionsin which concentrations of about 1 nanomolar or less can be used. Inother embodiment, methods utilize a concentration of about 200 picomolaror less and even a concentration of about 50 picomolar or less can beused in many circumstances.

The method can be carried out by addition of the siRNA compositions toany extracellular matrix in which cells can live provided that the siRNAcomposition is formulated so that a sufficient amount of the siRNA canenter the cell to exert its effect. For example, the method is amenablefor use with cells present in a liquid such as a liquid culture or cellgrowth media, in tissue explants, or in whole organisms, includinganimals, such as mammals and especially humans.

Expression of a target gene can be determined by any suitable method nowknown in the art or that is later developed. It can be appreciated thatthe method used to measure the expression of a target gene will dependupon the nature of the target gene. For example, when the target geneencodes a protein the term “expression” can refer to a protein ortranscript derived from the gene. In such instances the expression of atarget gene can be determined by measuring the amount of mRNAcorresponding to the target gene or by measuring the amount of thatprotein. Protein can be measured in protein assays such as by stainingor immunoblotting or, if the protein catalyzes a reaction that can bemeasured, by measuring reaction rates. All such methods are known in theart and can be used. Where the gene product is an RNA species expressioncan be measured by determining the amount of RNA corresponding to thegene product. The measurements can be made on cells, cell extracts,tissues, tissue extracts or any other suitable source material.

The determination of whether the expression of a target gene has beenreduced can be by any suitable method that can reliably detect changesin gene expression. Typically, the determination is made by introducinginto the environment of a cell undigested siRNA such that at least aportion of that siRNA enters the cytoplasm and then measuring theexpression of the target gene. The same measurement is made on identicaluntreated cells and the results obtained from each measurement arecompared.

The siRNA can be formulated as a pharmaceutical composition whichcomprises a pharmacologically effective amount of a siRNA andpharmaceutically acceptable carrier. A pharmacologically ortherapeutically effective amount refers to that amount of a siRNAeffective to produce the intended pharmacological, therapeutic orpreventive result. The phrases “pharmacologically effective amount” and“therapeutically effective amount” or simply “effective amount” refer tothat amount of a RNA effective to produce the intended pharmacological,therapeutic or preventive result. For example, if a given clinicaltreatment is considered effective when there is at least a 20% reductionin a measurable parameter associated with a disease or disorder, atherapeutically effective amount of a drug for the treatment of thatdisease or disorder is the amount necessary to effect at least a 20%reduction in that parameter.

The phrase “pharmaceutically acceptable carrier” refers to a carrier forthe administration of a therapeutic agent. Exemplary carriers includesaline, buffered saline, dextrose, water, glycerol, ethanol, andcombinations thereof. For drugs administered orally, pharmaceuticallyacceptable carriers include, but are not limited to pharmaceuticallyacceptable excipients such as inert diluents, disintegrating agents,binding agents, lubricating agents, sweetening agents, flavoring agents,coloring agents and preservatives. Suitable inert diluents includesodium and calcium carbonate, sodium and calcium phosphate, and lactose,while corn starch and alginic acid are suitable disintegrating agents.Binding agents may include starch and gelatin, while the lubricatingagent, if present, will generally be magnesium stearate, stearic acid ortalc. If desired, the tablets may be coated with a material such asglyceryl monostearate or glyceryl distearate, to delay absorption in thegastrointestinal tract. The pharmaceutically acceptable carrier of thedisclosed dsRNA composition may be micellar structures, such as aliposomes, capsids, capsoids, polymeric nanocapsules, or polymericmicrocapsules.

Polymeric nanocapsules or microcapsules facilitate transport and releaseof the encapsulated or bound dsRNA into the cell. They include polymericand monomeric materials, especially including polybutylcyanoacrylate. Asummary of materials and fabrication methods has been published (seeKreuter, 1991). The polymeric materials which are formed from monomericand/or oligomeric precursors in the polymerization/nanoparticlegeneration step, are per se known from the prior art, as are themolecular weights and molecular weight distribution of the polymericmaterial which a person skilled in the field of manufacturingnanoparticles may suitably select in accordance with the usual skill.

Suitably formulated pharmaceutical compositions of this invention can beadministered by any means known in the art such as by parenteral routes,including intravenous, intramuscular, intraperitoneal, subcutaneous,transdermal, airway (aerosol), rectal, vaginal and topical (includingbuccal and sublingual) administration. In some embodiments, thepharmaceutical compositions are administered by intravenous orintraparenteral infusion or injection.

In general a suitable dosage unit of siRNA will be in the range of 0.001to 0.25 milligrams per kilogram body weight of the recipient per day, orin the range of 0.01 to 20 micrograms per kilogram body weight per day,or in the range of 0.01 to 10 micrograms per kilogram body weight perday, or in the range of 0.10 to 5 micrograms per kilogram body weightper day, or in the range of 0.1 to 2.5 micrograms per kilogram bodyweight per day. Pharmaceutical composition comprising the siRNA can beadministered once daily. However, the therapeutic agent may also bedosed in dosage units containing two, three, four, five, six or moresub-doses administered at appropriate intervals throughout the day. Inthat case, the siRNA contained in each sub-dose must be correspondinglysmaller in order to achieve the total daily dosage unit. The dosage unitcan also be compounded for a single dose over several days, e.g., usinga conventional sustained release formulation which provides sustainedand consistent release of the siRNA over a several day period. Sustainedrelease formulations are well known in the art. In this embodiment, thedosage unit contains a corresponding multiple of the daily dose.Regardless of the formulation, the pharmaceutical composition mustcontain siRNA in a quantity sufficient to inhibit expression of thetarget gene in the animal or human being treated. The composition can becompounded in such a way that the sum of the multiple units of siRNAtogether contain a sufficient dose.

Data can be obtained from cell culture assays and animal studies toformulate a suitable dosage range for humans. The dosage of compositionsof the invention lies within a range of circulating concentrations thatinclude the ED₅₀ (as determined by known methods) with little or notoxicity. The dosage may vary within this range depending upon thedosage form employed and the route of administration utilized. For anycompound used in the method of the invention, the therapeuticallyeffective dose can be estimated initially from cell culture assays. Adose may be formulated in animal models to achieve a circulating plasmaconcentration range of the compound that includes the IC₅₀ (i.e., theconcentration of the test compound which achieves a half-maximalinhibition of symptoms) as determined in cell culture. Such informationcan be used to more accurately determine useful doses in humans. Levelsof dsRNA in plasma may be measured by standard methods, for example, byhigh performance liquid chromatography.

In a further aspect, the present invention relates to a method for TGSin a mammalian, including human, cell. The method comprises introducingthe siRNA into the appropriate cell. The term “introducing” encompassesa variety of methods of introducing DNA into a cell, either in vitro orin vivo. Such methods include transformation, transduction,transfection, and infection. Vectors are useful and preferred agents forintroducing DNA encoding the siRNA molecules into cells. The introducingmay be accomplished using at least one vector. Possible vectors includeplasmid vectors and viral vectors. Viral vectors include retroviralvectors, lentiviral vectors, or other vectors such as adenoviral vectorsor adeno-associated vectors. In one embodiment, the DNA sequences areincluded in separate vectors, while in another embodiment, the DNAsequences are included in the same vector. The DNA sequences may beinserted into the same vector as a multiple cassettes unit. Alternatedelivery of siRNA molecules or DNA encoding siRNA molecules into cellsor tissues may also be used in the present invention, includingliposomes, chemical solvents, electroporation, viral vectors,pinocytosis, phagocytosis and other forms of spontaneous or inducedcellular uptake of exogenous material, as well as other delivery systemsknown in the art.

Suitable promoters include those promoters that promote expression ofthe interfering RNA molecules once operatively associated or linked withsequences encoding the RNA molecules. Such promoters include cellularpromoters and viral promoters, as known in the art. In one embodiment,the promoter is an RNA Pol III promoter, which preferably is locatedimmediately upstream of the DNA sequences encoding the interfering RNAmolecule. Various viral promoters may be used, including, but notlimited to, the viral LTR, as well as adenovirus, SV40, and CMVpromoters, as known in the art.

In one embodiment, the invention uses a mammalian U6 RNA Pol IIIpromoter, and more preferably the human U6snRNA Pol III promoter, whichhas been used previously for expression of short, defined ribozymetranscripts in human cells (Bertrand et al., 1997; Good et al., 1997).The U6 Pol III promoter and its simple termination sequence (four to sixuridines) were found to express siRNAs in cells. Appropriately selectedinterfering RNA or siRNA encoding sequences can be inserted into atranscriptional cassette, providing an optimal system for testingendogenous expression and function of the RNA molecules.

In a further aspect, the invention provides a method for TGS in amammalian, including human, cell comprising introducing into the cellDNA sequences encoding a sense strand and an antisense strand of ansiRNA, which is specific for a target sequence in the gene to besilenced, preferably under conditions permitting expression of the siRNAin the cell, and wherein the siRNA directs methylation of said gene ofinterest. In an embodiment, methylation is directed to a sequence in thepromoter region of the gene. Alternately, methylation is directed to asequence in the coding region. Target sequences can be any sequence in agene that has the potential for methylation. In a preferred embodiment,the target sequences may contain CpG islands. The directed methylationcan lead to inactivation of the gene. To target more than one sequencein the gene (such as different promoter region sequences and/or codingregion sequences), separate siRNA-encoding DNA sequences specific toeach targeted gene sequence can be introduced simultaneously into thecell. In addition, cells may be exposed to multiple siRNAs that targetmultiple sequences in the gene.

Once a target sequence or sequences have been identified for methylationin accordance with the invention, the appropriate siRNA can be produced,for example, either synthetically or by expression in cells. In a oneembodiment, the DNA sequences encoding the sense and antisense strandsof the siRNA molecule can be generated by PCR. In another embodiment,the siRNA encoding DNA is cloned into a vector, such as a plasmid orviral vector, to facilitate transfer into mammals. In anotherembodiment, siRNA molecules may be synthesized using chemical orenzymatic means.

To facilitate nuclear retention and increase the level of methylation,the sense and antisense strands of the siRNA molecule may be expressedin a single stranded form, for example as a stem loop structure, asdescribed above. Alternatively, or in concomitance, the factor(s)involved in the active cellular transport of siRNA's, such as Exportin5, may be downregulated employing synthetic siRNA, antisense, ribozymes,or any other nucleic acid, antibody or drug, proven to be effective indownregulating the gene(s) of interest.

The procedure for a PCR-based approach is depicted schematically in FIG.1 and illustrated in Example 1. In one embodiment, a universal primerthat is complementary to the 5′ end of the human U6 promoter is used ina PCR reaction along with a primer(s) complementary to the 3′ end of thepromoter, which primer harbors appended sequences which arecomplementary to the sense or antisense siRNA genes (FIG. 1A). The senseor antisense sequences are followed by a transcription terminatorsequence (Ter), which is preferably a stretch of 4-6 deoxyadenosines,and more preferably a stretch of 6 deoxyadenosines, and by a shortadditional “stuffer-tag” sequence that may include a restriction sitefor possible cloning at a later stage. The resulting PCR productsinclude the U6 promoter sequence, the siRNA sense or antisense encodingsequence, a terminator sequence, and a short tag sequence at the 3′terminus of the product.

In another embodiment, both the sense and antisense sequences can beincluded in the same cassette (FIG. 1B, 1C and 1D). In this case anucleotide loop, preferably containing 9 nucleotides, is insertedbetween the sense and antisense siRNA sequences. The resulting singlePCR product includes the U6 promoter, the siRNA sense and antisenseencoding sequences in the form of a stem-loop, the Pol III terminatorsequence, and the “stuffer” tag sequence (FIG. 1D). To construct thiscassette two 3′ primers are used. The first PCR reaction employs the 5′U6 universal (or “common”) primer and a 3′ primer complementary to aportion of the U6 promoter, followed by sequences complementary to thesiRNA sense encoding sequence and the 9 nt. loop (FIG. 1B). Preferablyone microliter of this first reaction is re-amplified in a second PCRreaction that employs the same 5′ U6 primer and a 3′ primer harboringsequences complementary to the 9 nt. loop appended to the antisensestrand, Ter and “stuffer” tag sequence (FIG. 1B).

In another embodiment, a one step PCR reaction is conducted with asingle 3′ primer that harbors the sense, loop, antisense, Ter and“stuffer” tag sequences (FIG. 1C).

The practice of the present invention employs, unless otherwiseindicated, conventional techniques of chemistry, molecular biology,microbiology, recombinant DNA, genetics, immunology, cell biology, cellculture and transgenic biology, which are within the skill of the art.See, e.g., Maniatis et al., Molecular Cloning (Cold Spring HarborLaboratory Press, Cold Spring Harbor, N.Y., 1982); Sambrook et al.,Molecular Cloning, 2nd Ed. (Cold Spring Harbor Laboratory Press, ColdSpring Harbor, N.Y., 1989); Sambrook and Russell, Molecular Cloning, 3rdEd. (Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y.,2001); Ausubel et al., Current Protocols in Molecular Biology (JohnWiley & Sons, updated through 2005); Glover, DNA Cloning (IRL Press,Oxford, 1985); Anand, Techniques for the Analysis of Complex Genomes,(Academic Press, New York, 1992); Guthrie and Fink, Guide to YeastGenetics and Molecular Biology (Academic Press, New York, 1991); Harlowand Lane, Antibodies, (Cold Spring Harbor Laboratory Press, Cold SpringHarbor, N.Y., 1998); Jakoby and Pastan, 1979; Nucleic Acid Hybridization(B. D. Hames & S. J. Higgins eds. 1984); Transcription And Translation(B. D. Hames & S. J. Higgins eds. 1984); Culture Of Animal Cells (R. I.Freshney, Alan R. Liss, Inc., 1987); Immobilized Cells And Enzymes (IRLPress, 1986); B. Perbal, A Practical Guide To Molecular Cloning (1984);the treatise, Methods In Enzymology (Academic Press, Inc., N.Y.); GeneTransfer Vectors For Mammalian Cells (J. H. Miller and M. P. Calos eds.,1987, Cold Spring Harbor Laboratory); Immunochemical Methods In Cell AndMolecular Biology (Mayer and Walker, eds., Academic Press, London,1987); Handbook Of Experimental Immunology, Volumes I-IV (D. M. Weir andC. C. Blackwell, eds., 1986); Riott, Essential Immunology, 6th Edition,(Blackwell Scientific Publications, Oxford, 1988); Hogan et al.,Manipulating the Mouse Embryo, (Cold Spring Harbor Laboratory Press,Cold Spring Harbor, N.Y., 1986); Westerfield, M., The zebrafish book. Aguide for the laboratory use of zebrafish (Danio rerio), 4th Ed., (Univ.of Oregon Press, Eugene, Oreg., 2000).

EXAMPLES

The present invention can be described by reference to the followingExamples, which are offered by way of illustration and are not intendedto limit the invention in any manner. Standard techniques well known inthe art or the techniques specifically described below were utilized.

Furthermore, the following summary of the Examples is not intended to belimiting to each respective Example and full details can be found in therespective priority documents. Examples 1-6 (USSN 10/446,635) Examples7-12 (USSN 60/683,782) Examples 13-18 (this application) siRNA inducesDNA methylation siRNA induces methylation of histones Ago1 recruitshistone methyltransferase associated w/target gene Reduces or increasestarget gene Reduces target gene expression Reduces target geneexpression. Ago1 expression directs siRNA mediated TGS siRNA ˜21-28 bps,19-23 bps, siRNA ˜18-29 bps, ˜18-23 bps, 21-26 nt noncoding RNAs 21 bps,24-28 bps, 26 bps, 28 bps ˜18-21 bps siRNA complementary to RASSF1AAnti-sense siRNA EF52 binding to DNMT3A Ago1 associates w/targetpromoters (CCR5 region directs methylation of directs methylation ofhistones related and RASSF1A) via an interaction with RASSF1A genepromoter leading to to target gene leading to reduction of RNAPIIleading to TGS reduced RASSF1A expression target gene expressionTransitional silencing complex (TSC): Ago1, TRBP2, siRNA and possiblychromatin remodeling factors

Example 1

Expression of Short Hairpin RNAs Complementary to Regions of RASSF1A

This example demonstrates expression of short hairpin RNAs that arecomplementary to regions of a human tumor suppressor gene RASSF1A. Theconsequences of this expression were monitored by determining thepatterns of DNA methylation in the promoter and part of the codingregion of this gene, which is also susceptible to methylation in cancercells. The DNA sequence of the RASSF1A gene is depicted below:

RASSF1A Promoter (SEQ ID NO:2): ggggctctgc gagagcgcgc ccagccccgccttcgggccc cacagtccct gcacccaggt ttccattgcg cggctctcct cagctccttcccgccgccca gtctggatcc tgggggag gc gctgaagtcg gggcccgcc c tgtggccccgcccggcccgc gcttgct

 gcccaaagcc

RASSF1A transcript (SEQ ID NO:3): agcgaagcac gggcccaaCC GGgccatgtc gggggagcct gagctcattg agctgcggga gctggcaccc gctgggcgcg ctgggaagggccgcacccgg ctggagcgtg ccaacgcgct gcgcatcgcg cggggcaccg cgtgcaaccccacacggcag ctggtccctg gccgtggcca ccgcttccag cccgcggggc ccgccacgcacacgtggtgc gacctctgtg gcgacttcat ctggggcgtc gtgcgcaaag gcctgcagtgcgcgcgtgag tagtggcccc gcgcgcctac

-   agc is where transcription probably starts-   atg is the methionine codon-   The bolded sequences were targeted by siRNAs of the invention.

PCR reactions are performed using a plasmid containing the human U6promoter as template to yield U6 transcription cassettes expressingsiRNAs. The 5′ oligonucleotide (5′U6 universal primer) is complementaryto 29 nucleotides at the 5′ end of the U6 promoter (bold italicsindicate the nucleotides complementary to those on the promoter).

5′U6 Mlu primer: (SEQ ID NO: 4) 5′ AATCGA ACGCGT

 3′           Mlu I             U6

This U6 common 5′ primer, used for all PCR steps, binds to the 5′ end ofthe U6 promoter and includes an Mlu I restriction site for cloningpurposes. The 3′ oligonucleotides, which contain either the sense,antisense, or both siRNA-coding sequences (siDNAs), are depicted in FIG.1 and described herein. The last 20 nucleotides at the 3′ end of all 3′PCR primers are complementary to the last 20 nucleotides of the U6promoter which is: 5′GTGGAAAGG ACGAAACACCG3′ (SEQ ID NO:5). All PCRreactions were carried out as follows: 1 min. at 94° C., 1 min. at 55°C. and 1 min. at 72° C. for 30 cycles. The PCR products can be directlytransfected into cells (e.g., with prior cloning into an expressionvector), in which event the PCR primers can be kinased withnon-radioactive ATP prior to amplification and purified on Quiagencolumns prior to using them in the PCR reactions. The PCR products alsocan be purified on Quiagen columns.

The 3′ primers used to make siRNA expression cassettes are depictedbelow:

Primers used to make PCR products encoding siRNA's complementary to thepromoter region of the RASSF1A gene:

3′PR 1 (SEQ ID NO:6) 5′CTACACAAA GGCGGGCCCCGACTTCAGCGC  GGTGTTTCGTCCTTTCCACAA 3′     loop         si-sense       +1         U6

3′PR 2 (SEQ ID NO:7) 5′AACTC GAATTC AAAAAA GCGCTGAAGTCGGGGCCCGCCCTACACAAA 3′          EcoRI   Ter.     si-antisense         Loop

Primers used to make PCR products encoding siRNA's complementary to thetranscribed region of the RASSF1A gene:

3′TR 1 (SEQ ID NO:8) 5′CTACACAAA CGACATGGCCCGGTTGGGCCC   GGTGTTTCGTCCTTTCCACAA 3′    loop          si-sense        +1          U6

3′TR 2 (SEQ ID NO:9) 5′AACTC GAATTC AAAAAA GGGCCCAACCGGGCCATGTCGCTACACAAA 3′          EcoRI Ter.        si-antisense       Loop

Example 2

HeLa Cells Stably Transfected with the siRNA Expression Constructs

HeLa cells, which include in their genome the RASSF1A gene, were stablytransfected with the siRNA expression constructs produced by the methodshown above. The final siRNAs-containing PCR products were digested withMluI and EcoRI and cloned in the same sites of the pcDNA3.1 vector(Invitrogen) for expression in the mammalian cells. Digestion ofpcDNA3.1 with MluI and EcoRi allows the replacement of the CMV promoterwith the U6 siRNA cassettes. The Neomycin gene is the marker gene forselection in mammalian cells. Cells were selected for G418 resistance.Cells were monitored either in mixed population or clones of transfectedcells.

Stable cell lines expressing all different siRNAs and 8 individualsingle clones for each of the siRNA expressing cells have thus far beenobtained.

Example 3

Methods to Determine Methylation

Bisulfite: In the mixed cell population, genomic DNA was isolated andtreated with bisulfite, which changes unmethylated cytosines tothymidines. Methylated cytosines remain as cytosine. Thus, if the siRNAsdirect methylation of the targeted sequences of the RASSF1A shown inExample 1, these DNAs will not be modified by bisulfite in themethylated region.

MSP Assay: PCR primers specific for either methylated or unmethylatednucleotides were used in PCR reactions in accordance with theMethylation-specific PCR assay (MSP assay) described in Herman et al.Results showed that the siRNA that targets the promoter region and thesiRNA that targets the RASSF1A transcript, were directing methylation ofthe RASSF1A gene. The MSP assay is sensitive and specific formethylation of virtually any block of CpG sites in a CpG island. Theassay uses primers designed to distinguish methylated from unmethylatedDNA in bisulfite-modified DNA, taking advantage of the sequencedifferences resulting from bisulfite modification. Unmodified DNA or DNAincompletely reacted with bisulfite can also be distinguished, sincemarked sequence differences exist between these DNAs.

FIG. 2 shows results of the MSP analysis of the RASSF1A promoter insiRNA transfected cells. In the figure, H₂O represents a water controlused in the PCR reactions. The following additional abbreviations werealso used:

pcDNA: Cells transfected only with the vector (no siRNA)

siRASSF1Amut: Cells transfected with the mutant siRNA vector

siRASSF1Aprom: Cells transfected with the siRNA vector directed againstthe RASSF1A promoter sequences

siRASSF1Atx: Cells transfected with the siRNA vector directed againstthe RASSF1A transcript

Melanoma: a control for RASSF1A methylation. This is DNA from a melanomatumor, which is methylated in the RASSF1A promoter.

M, size markers

m, MSP done with primers specific for a methylated RASSF1A promoter

u, MSP done with primers specific for an unmethylated RASSF1A promoter

The following primers were used in the MSP reaction: methylatedDNA-specific primers, M210 (5′ GGGTTTTGCGAGAGCGCG 3′) (SEQ ID NO:10) andM211 (5′GCTAACAAACGC GAACCG 3′) (SEQ ID NO:11) or unmethylatedDNA-specific primers UM240 (5′ GGGGTTTTGT GAGAGTGTGTTTAG 3′) (SEQ IDNO:12) and UM241 (5′ TAAACACTAACAAACACAAAC CAAAC 3′) (SEQ ID NO:13)(Liu, L. et al., 2002).

Restriction Analysis: Restriction analyses with an enzyme thatrecognizes only the methylated sequence (BstU1), also confirmed thepresence of methylated sites in the RASSF1A gene.

Sequencing: Specific deoxynucleotide primed sequencing revealed that 14out of 17 potential methylation sites analyzed in the RASSF1A gene weremethylated in cell populations expressing the siRNA directed against theRASSF1A promoter, and 17 out of 17 sites were methylated in cellsexpressing the siRNA directed against a CpG island in the RASSF1Atranscript. Results are shown in FIG. 3. The level of methylation in thepromoter region was higher in some of the single clones analyzed.Specific integration sites of siRNAs in the cellular genome (by usingthe appropriate delivering vector) could be used to achieve completepromoter methylation.

Sequence data were obtained by sequencing of the PCR products obtainedfrom the MSP reactions of Example 4 (FIG. 2). In FIG. 3, sampledesignation is the same as in FIG. 2. FIG. 3 shows the RASSF1A promotersequence relative to the ATG translation start site (i.e. −30 indicates30 nucleotides upstream). Open circles represent unmethylated cytosinesat CG sequences. Closed circles indicate methylated cytosines at CGsequences.

Example 4

Negative Control

As a negative control, DNA was extracted from cells expressing a mutatedsiRNA, was analyzed, and showed no effects on the methylation of theRASSF1A gene. In this analysis, PCR products were produced as describedin Example 1, but using the 3′ primers shown below. For the mutant therewere two transversions (CCGG to GGCC) and one transition (C to T) tomake sure it would be inactive.

Mutant primers against transcribed region:

3′MT 1 (SEQ ID NO:14)               (c)   (ccgg)5′CTACACAAA CGATATGGCGGCCTTGGGCC C   GGTGTTTCGTCCTTTCCACAA 3′     loop         si-sense        +1          U6

3′MT 2 (SEQ ID NO:15) 5′AACTC GAATTC AAAAAA GGGCCCAAGGCCGCCATATCGCTACACAAA 3′          EcoRI   Ter.       si-antisense      Loop

Example 5

Reduction of RASSF1A Intracellular Expression in Cells Transfected withshRNAs Directed Against Promoter Sequences

FIG. 4 shows the reduction of RASSF1A RNA transcripts detected byreverse transcriptase PCR (RT-PCR) reactions. Hela cells weretransfected with shRNAs directed against promoter sequences of RASSF1A.Cells were collected after 48-56 hr. and the RNA was extracted using RNASTAT60 as suggested by the manufacturer. Quantitative PCR reactions wereperformed by preparing 100 μl PCR mixes containing standard PCR buffer,dNTPs, 1 μg of each RNA sample, and two 3′ primers specific to eitherthe RASSF1A transcript or to the GAPDH cellular gene. GAPDH is used asan internal control to verify the integrity and amount of RNA analyzedin each reaction. After the samples were heated at 80° C. for 1 minuteand slow cooled to room temperature, they were thoroughly mixed anddivided into two 50 μl aliquots. 1-2 units of reverse transcriptase wereadded to half of the reactions while the other half were used ascontrols to exclude DNA contaminations. All samples were placed at 37°C. for 5 minute to complete the extension reactions. Following theextensions (and cDNA synthesis) the samples were thoroughly mixed anddivided once again into two 25 μl aliquots. The specific 5′ primers forthe RASSF1A or the GAPDH were added to the 25 μl aliquots and the PCRreactions were completed as for the methylation-specific PCR assay.

As shown in FIG. 4, representative clonal cell lines from cellstransfected with the 21 nucleotides shRNAs directed against the RASSF1Apromoter (21c1, 21c2, 21c3), and the Hela cell population transfectedwith a 28 nucleotides shRNA (sh28) were analyzed for decreased RNAexpression. Clonal cell lines tranfected with the shRNA mutant (Mtc1,Mtc2, Mtc3) were also analyzed as controls. After normalization with theGAPDH internal control, a clear and specific RASSF1A RNA down-regulationcan be detected in two of the three clones expressing shRNA directedagainst promoter sequences, but in none of the mutant shRNA clones usedas controls. The -RT controls showed no DNA contamination. These resultsindicate that specific shRNA methylation of the RASSF1A promoter resultsin down-regulation of the intracellular RASSF1A transcripts.

Example 6

Decreased Expression of RASSF1A Transcripts

Several clonal HeLa cell lines transfected with 28 nucleotides shRNAsdirected against the promoter sequences were analyzed by ReverseTranscriptase dependent PCRs as described in Example 8. The resultsshown in FIG. 5 show decrease expression of RASSF1A transcripts in manyof the clones analyzed. Similar results were obtained by expressing theshRNAs from lentiviral vector backbones (not shown), which may be themethod of choice (but not the only method) for long-term expression ofshRNAs and gene silencing. The results obtained with the clonal celllines transfected with the various shRNAs are summarized in FIG. 6.

The above demonstrates the invention's utility for, among other things,designing and using siRNAs to direct DNA methylation in either apromoter region or certain coding region of a gene. Directing promotermethylation of a gene by targeting siRNAs against CpG islands of RNAtranscripts should be a potent inhibitor of intracellular geneexpression.

Example 7

Methods Used for Examples 8-12

Chromatin immunoprecipitation assay (CHiP): Chromatinimmunoprecipitation was performed on 4.0×10⁶ 293T transfected with siRNAEF52 or control CCR5 (10 NM using MPG 3 μl/ml of media) (Morris et al.,2004b). Forty-eight hours following transfection cultures were collectedand ChiP assay performed as described (Strahl-Bolsinger et al., 1997).Cultures were specifically probed with anti-dimethyl-Histone H3 (Lys9)and anti-trimethyl-Histone H3 (Lys27) (Upstate catalog #07-441 and07-449, respectively). The final elutes were assayed using PCR 30 cyclesof 94:55:72° C. with primers 803 and 804 which specifically overlap thetargeted EF1 alpha promoter (Morris et al., 2004a) and quantitated usingthe IDV values determined from analysis with the Alpha Innotech.

Detection of Flag-tagged proteins in biotin labeled siRNA pulldowns: Atotal of 4.0×10⁶ 293T cells were transfected with 15 μg of one of 9Flag-tagged expression vectors (DNMT-1, 3A, 3A2, 3B1, 3B2, HP1-alpha,HP1-beta, HP1-gamma, or the negative control helicase Prp2) usingLipofectamine 2000™m. All DNMTs were supplied by A. Riggs and all HP1swere a gift R. Losson (Nielsen et al., 2001). (2) Forty-eight hourslater the cell lysates (cytoplasmic and nuclear fractions) were isolatedeach in 500 μl of lysis buffer (1 mM PMSF, 20 units RNasin, 10 mMTris-HCl pH 7.4, 10 mM NaCl, 3 mM MgCl₂, 0.1 mM DTT, and 0.5% NP40). (3)A mixture of 125 μl of cytoplasmic and nuclear fractions were incubatedfor 3 hrs at 4° C. with 500 nM 5′Biotin end-labeled EF52. Next DynalAvidin/magnetic beads (7×10⁷ beads) were washed in lysate buffer andthen added directly to the siRNA/Flag-Tagged containing lysates andincubated at 4° C. for 1 hr. The siRNA/Flag complexes were pulled-downby magnetic bead binding and washed 5×'s in lysate buffer. Finally, thebound protein complexes were eluted from the Avidin-biotin boundbeads/siRNA by incubation in 100 μl of elution buffer (Tris-Cl pH 6.0, 1mM EDTA, 2.0 M NaCl, 0.5 M MgCl₂) at 55° C. for 5 minutes. The elutedprotein complexes were electrophoresed in denaturing PAGE and subjectedto western blot analysis with an anti-Flag antibody.

Detection of siRNAs from Flag-tag pulldowns: Flag-tagged DNMT1 and 3Awere as described previously. The cell lysates/extracts were thenincubated with either the siRNA EF52, sense (S) or antisense (AS) EF52(500 nM) for 3 hrs at 4° C. Next, a flag-tag immunoprecipitation wasperformed for each DNMT complex containing the putative bound siRNAsfollowed by deproteinization of the complex, phenol/chloroformextraction, and release of the bound siRNA. Detection of the releasedsiRNA, either sense or antisense, was performed according to Weinberg etal. (2006).

ChiP/biotin-RNA co-immunoprecipitation: A total of 4.0×10⁶ 293T cellswere plated and 24 hrs later transfected with 100 nM EF52 biotin labeledsiRNA (antisense alone) using MPG (3 μl/ml of media). Forty eight hoursfollowing the siRNA/MPG transfection cultures were collected and a ChiPassay performed as described previously with a slight modification.Following the immunoprecipitation with the H3K27 tri-methyl specificantibody the elutes were incubated with 100 μl (6-7×10⁸ beads/ml) ofDynabeads™ M-280 Streptaviden pre-washed in 2× wash buffer (50 mM TrisHCl, 400 mM NaCl pH 7.4). The elute/bead slurry was incubated at 4° C.for 15 minutes on an orbital shaker followed by capture with a magneticbead separator. The captured beads were washed 3 times in 2× wash bufferand then eluted in 100 μl of 2× elute buffer (10 mM Tris-HCL (pH 6.0), 1mM EDTA and 2.0 M NaCl) at 65° C. for 5 minutes. The resultant eluteswere then reverse cross-linked and DNA recovered by phenol/chloroformextraction followed by PCR 35 cycles of 94:55:72° C. for the EF1 alphapromoter with primers 803 and 804 (Morris et al., 2004a).

ChiP/DNMT3A-Flag/biotin-RNA co-immunoprecipitation: A total of 4.0×10⁶²⁹³T cells were plated and 24 hrs later transfected with 100 nM EF52biotin labeled siRNA (antisense or sense alone) using MPG (3 μl/ml ofmedia). Forty eight hours following the sense or antisense siRNA/MPGtransfection cultures were collected and a ChiP assay performed asdescribed previously with a slight modification. Following theimmunoprecipitation with the H3K27 tri-methyl specific antibody theelutes were incubated overnight with 40 μl of EZVIEW™ Red anti-Flag M2affinity gel beads (Sigma™). Next the bound beads were washed 3 timeswith TBS-Mod Buffer (50 mM Tris HCl, 400 mM NaCl pH 7.4) and then elutedby competition with 3× Flag-peptide 15 μg. The resultant elutes werethen transferred to 100 μl (6-7×10⁸ beads/ml) of Dynabeads™ M-280Streptaviden pre-washed in 2× wash buffer (50 mM Tris HCl, 400 mM NaClpH 7.4). The elute/bead slurry was incubated at 4° C. for 15 minutes onan orbital shaker followed by capture with a magnetic bead separator.The captured beads were washed 3 times in 2× wash buffer and then elutedin 100 μl of 2× elute buffer (10 mM Tris-HCL (pH 6.0), 1 mM EDTA and 2.0M NaCl) at 65° C. for 5 minutes. The resultant elutes were then reversecross-linked and DNA recovered by phenouchloroform extraction followedby PCR 35 cycles of 94:55:72° C. for the EF1 alpha promoter with primers803 and 804 (Morris et al., 2004a).

HIV-1 U3 LTR Targeting: Small interfering RNAs were constructedfollowing established protocols (Ambion Silencer™). EF1-alpha siRNAtarget sites were: EF52 5′-AAG GTG GCG CGG GGT AAA CTG-3′ (SEQ IDNO:16), and the control GFP mRNA specific 5′-AAC GAT GCC ACC TAC GGCAAG-3′ (kit control; SEQ ID NO:17), and negative control CCR5 specific5′-AAT TCT TTG GCC TGA ATA ATT-3′ (SEQ ID NO:18). Synthetic 5′ endbiotin labeled EF52 sense (S) 5′-CCA CCG CGC CCC AUU UGA CAA-3′ (SEQ IDNO:19), antisense (AS) 5′AAG GUG GCG CGG GGU AAA CUG-3′ (SEQ ID NO:20)and urnodified sense and antisense siRNAs used in co-immunoprecipitationassays were constructed at the City of Hope Beckman Research InstituteDNA, RNA and Peptide Synthesis Facility. To generate the sense/antisense(S/AS) biotin end-labeled siRNAs equivalent volumes of 100 μM of thesense and antisense siRNAs were mixed together and incubated at 65° C.for 5 minutes followed by a 5 minute incubation on ice. HIV-1 U3 LTRtargeted siRNAs were constructed and cloned into pCR4-TOPO (Invitrogen™)by PCR using the 5′ U6+1 as described (Lee et al., 2002) with 3′primers; LTR-247(S): 5′AAA AAA AAG TGT TAG AGT GGA GGT TTG CGG TGT TTCGTC CTT TCC ACA A-3′ (SEQ ID NO:21), LTR-362(AS): 5′AAA AAA AAG AAA GTCCCC AGC GGA AAG CGG TGT TTC GTC CTT TCC ACA A-3′ (SEQ ID NO:22),GFP(AS): 5′AAA AAA AAC GAT GCC ACC TAC GGC AAG CGG TGT TTC GTC CTT TCCACA A-3′ (SEQ ID NO:23), GFP(S): 5′AAA AAA AAC TTG CCG TAG GTG GCA TCGCGG TGT TTC GTC CTT TCC ACA A-3′ (SEQ ID NO:24), LTR-247c(AS): 5′-AAAAAA AAG TAT TAA AGT GGA AGT TTG CGG TGT TTC GTC CTT TCC ACA A-3′ (SEQ IDNO:25), LTR-247c(S): 5′-AAA AAA AAC AAA CTT CCA CTT TAA TAC GGT GTT TCGTCC TTT CCA CAA-3′ (SEQ ID NO:26), LTR-362c(AS): 5′-AAA AAA AAC TTT CCACTG GGG CGT TCC GGT GTT TCG TCC TTT CCA CAA-3′ (SEQ ID NO:27),LTR-362c(S): 5′-AAA AAA AAG GAA CGC CCC AGT GGA AAG CGG TGT TTC GTC CTTTCC ACA A-3′ (SEQ ID NO:28). The resultant clones were co-transfectedwith the HIV-1 Tat expression plasmid pTatdsRed2 (Unwalla et al., 2004)into 4.0×10⁵ U3-Luciferase indicator cells TZM-BI obtained through theNIH AIDS Research and Reference Reagent Program, Division of AIDS,NIAID, (Wei et al., 2002; Platt et al., 1998). Twenty-four and/or fortyeight hours later luciferase expression was determined using theDual-LuciferaseR Reporter Assay System (Promega™) and a Veritas™microplate luminometer from Turner Biosystems following the manufacturesprotocols.

Alpha-Amanatin mediated suppression of siRNA induced TGS: Chromatinimmunoprecipitation was performed on 4.0×10⁶ 293T transfected with siRNAEF52 or control CCR5 (10 nM using MPG 3 μl/ml of media) (Morris et al.,2004b). Twenty-four hours following MPG mediated siRNA transfectioncultures ½ of the cultures were exposed to Alpha amanatin (0.05 μg/ml)and 24 hrs later collected and ChiP assay performed as described(Strahl-Bolsinger et al. 1997). Cultures were specifically probed withanti-dimethyl-Histone H3 (Lys9)(Upstate catalog #07-441 and -7-449respectively). The final elutes were assayed using PCR 30 cycles of94:55:72° C. and 15,15 and 30 seconds respectively with primers 803 and804 which specifically overlap the targeted EF1 alpha promoter (Morriset al., 2004a).

Example 8

EF52 siRNA Induces Histone Methylation

siRNA EF52 is homologous to a sequence in the EFLA promoter and has beenshown to induce TGS of endogenous EF1A (Morris et al., 2004b). The EF52mediated TGS of endogenous EF1A was shown to involve both histone andDNA methylation. Moreover, silencing of promoters by DNA methylation hasbeen shown to be preceded by histone methylation (Mutskov andFelsenfeld, 2004). To investigate the histone methyl mark induced bysiRNA EF52 we transfected 293T cells with either EF52 or the controlCCR5 siRNA (Morris et al., 2004b) using the nuclear specific peptide MPG(Morris et la., 2003; Morris et al., 1997). EF52 treated culturesexhibited a pronounced increase in H3K9 and H3K27 methylation relativeto controls (FIG. 7A). Moreover, the induction of H3K9 methylation wascontingent on nuclear specific delivery of the EF52 siRNA (FIG. 7B).

Example 9

EF52 siRNA Pulldown DNMT3A

Transcriptional gene silencing by siRNAs in human cells involves somelevel of DNA methylation (Morris et al., 2004b; Kawasaki and Taira,2004), indicating that DNA methyltransferases might be involvedmechanistically in the observed silencing. To determine the mechanismunderlying previously observed TGS in human cells we developed an siRNApull-down assay (FIG. 8) and screened the binding potential of DNAmethyltransferases (DNMT) 1, 3A, 3A2, 3B1, 3B2, and heterochromatinproteins (HP1-alpha, beta, and gamma) to the promoter targeted EF52siRNAs (Table 1). Expression of each of the flag-tagged proteins wasdetected in the whole cell lysates with the exception of DNMT 3B1 andHP1-gamma, which had low to no expression (FIGS. 9A and 9B,respectively). Remarkably, when the whole cell lysates (FIGS. 9A-9B)were incubated in the presence of 5′ biotin end labeled EF52 siRNA andthe complexes pulled-down with avidin bound magnetic beads only DNMT 3A,3A2 and 3B2 were eluted (FIG. 9C). While the control Prp2, Mock, andDNMT-1 (MT1) showed no binding to the EF52 biotin labeled siRNAs (FIGS.9A-9C). The binding of DNMT3A was similar to previously reportedfindings of siRNAs binding in vitro to mouse DNMT3A (Jeffery andNakielny, 2004).

Next we wanted to determine if there was any strand specificity in theDNMT3A/EF52 siRNA binding, as strand specific binding could beindicative of the underlying mechanism of siRNA mediated TGS. Todetermine the specificity of binding we incubated biotin 5′ end labeledsense, antisense, sense/antisense and control non-biotin labeledsense/antisense siRNAs with DNMT1 and DNMT3A containing extracts (FIG.9D). Interestingly, the antisense strand of EF52 showed significantlyincreased binding potential that was comparable to thebiotin-sense/antisense treatment alone (FIG. 9E). A similar observationwas gained by performing a DNMT-Flag immunoprecipitation followed byprobing with radiolabelled siRNAs (sense or antisense) (FIG. 10). Thesedata suggest that the antisense strand may direct the observed siRNAmediated TGS through interactions with DNMT3A. TABLE 1 DNMTs and HP1sused in siRNA pull-down experiments. Flag-Tagged protein Function DNMT-1Maintenance DNA methyltransferase, also involved in (MT1) methylationduring embryogenesis (8) DNMT-3A De novo methylation, involved inmethylation during embryogenesis and transcriptional repression (8, 9)DNMT-3A2 De novo methyltransferase (8) DNMT-3B1 De novo methylation,involved in methylation during embryogenesis and transcriptionalrepression (8, 9) DNMT-3B2 Involved in methylation during embryogenesis(8) HP1-alpha Involved in transcriptional silencing by tethering DNA andbind core histones (10, 11) HP1-beta Involved in transcriptionalsilencing by tethering DNA and bind core histones (10, 11) HP1-gammaInvolved in transcriptional silencing by tethering DNA and bind corehistones (10, 11) PRP2 Negative control: RNA dependent RNA-ATpase (12,13)All flag-tagged DNMTs were a gift from A. Riggs (COH/BRI), HP1 (alpha,beta, gamma) were a gift R. Losson, Institute de Genetique et deBiologie Moleculaire et Cellulaire, France, and PRP2 was a gift from R JLin (COH/BRI).

Example 10

Histone Methylation and siRNA Specificity

Unlike RNA interference, transcriptional silencing in mammalian cells ismediated by a combination of chromatin modifications that includehistone deacetylation and cytosine DNA methylation (Bird and Wolffe,1999). Silencing by EF52 siRNA was completely relieved by treating thecells with TSA and 5′-AzaC, drugs that inhibit histone deacetylases andDNA methyltransferases respectively (Morris et al., 2004b) and datapresented here clearly shows H3K9 and H3K27 methylation is involved inthe observed siRNA induced TGS of EF1A. To explore the link betweenhistone 3 lysine methylation and siRNA specificity to the targetedpromoter we performed a ChiP/RNA co-immunoprecipitation assay (asdepicted in FIG. 8B). A ˜4.8 fold increase in detectable EF52 targetedpromoter relative to the no antibody control was observed inH3K27/antisense EF52 siRNA co-immunoprecipitates indicating H3K27,antisense siRNA EF52, and the targeted EF1A promoter EF1A co-localize invivo (FIG. 11A). The observation(s) that 1) siRNAs bind DNMT3A (FIG.9A-9D and Jeffery and Nakielny (2004)), 2) siRNA mediated TGS isreversible with the addition of both 5-AzaC and TSA (Morris et al.,2004b) and 3) Histone 3 lysine methylation (FIG. 7A) is present offerssome clues to the underlying complex involved in siRNA mediated TGS inhuman cells.

To investigate the core complex involved in siRNA EF52 mediated TGS weperformed a triple-immunoprecipitation assay (as depicted in FIG. 8C).This assay consists of first a CHiP for H3K27 followed by a Flag-TaggedDNMT3A immunoprecipitation and then a siRNA biotin/avidin pulldownfollowed by a PCR for the targeted promoter in the final elute.Interestingly, the antisense EF52 strand was enriched ˜2 and 3.5 foldrelative to the no antibody and sense alone controls respectively (FIG.11B). Notably the triple-immunoprecipitation was relatively inefficientwith the antisense elute containing only ˜7.6% of the control input(FIG. 11B).

Example 11

Antisense Strand of siRNA Directs TGS

The observation that the antisense strand of the siRNA is preferentiallydetected in the co-immunoprecipitation assays suggest that the antisensecan function alone to direct TGS. To determined if the antisense alonecan direct TGS we designed plasmids expressing from the U6 promotereither the antisense, sense, or both sense/antisense targeting the U3region of the HIV-1 LTR/promoter. Indeed, both U3 LTR specific siRNAs(Table 2) and remarkably, the antisense showed a profound and robustsuppression of U3 expressed luciferase relative to controls in TZM-B1cells (FIG. 12 and 11E). However, while TZM-B1 cells contain anintegrated lentiviral vector expressing luciferase from the HIV-1 LTRthey also contain the 3′ LTR (Wei et al., 2002). As such it is possiblethat some of the observed suppression was the result of the antisensesiRNAs binding the 3′ LTR and thus inhibiting luciferase expression in aPTGS based fashion. To determine if the antisense siRNAs can function toinduce TGS we transfected 1G5 cells containing the LTR expressing theluciferase with an SV40 poly-A (Aguilar-Cordorva et al., 1994).Interestingly, only the antisense LTR-247 siRNA (Table 2) induced TGS(FIG. 11). These data clearly suggest that the antisense strand of thesiRNA directs transcriptional silencing in human cells as well assuggest that fundamental differences in target site accessibility mightalso be present (i.e. LTR-362 overlaps the NF-kB binding site andLTR-247 does not, Table 2). TABLE 2 siRNAs used in the HIV-1 U3Targeting. Sequence (Target) siRNA (Position) (SEQ ID NO:) % GC 247(249-267 in LTR GTGTTAGAGTGGAGGTTTG (29) 47.4 of HIV subtype B) 362(354-372 in LTR CTTTCCGCTGGGGACTTTC (30) 57.9 of HIV subtype B) 247c(249-267 in LTR GTATTAAAGTGGAAGTTTG (31) 31.5 of HIV subtype C) 362c(354-372 in LTR CTTTCCACTGGGGCGTTCC (32) 63.2 of HIV subtype C) GFP(108-126 in GFP CGATGCCACCTACGGCAAG (33) 63.2 mRNA) R5 Control (787-805TTCTTTGGCCTGAATAATT (34) 31.6 in CCR5 mRNA)

Example 12

Transcription Required for siRNA Mediated TGS

The observation that the antisense strand of the siRNA is preferentiallyinvolved in siRNA mediated TGS suggest a mechanism that may be anantisense siRNA/RNA interaction (possibly non-coding RNA, personalcommunication R. Allshire) or an antisense siRNA/DNA interaction ispresent in the siRNA promoter targeting. The observation that onlyLTR-247 can mediate TGS of the U3 from HIV-1 LTR also supports apromoter accessibility or siRNA/DNA interaction. Regardless, bothpossibilities suggest that transcription may be required for theinitiation of siRNA mediated TGS. To determine if transcription isrequired for siRNA mediated TGS we performed EF52 (treatment) or CCR5(control) MPG mediated transfections and 24 hrs later treated ½ of thecultures with alpha amanatin (0.05 μg/ml) to inhibit RNA polymerase II(Pol-II) and then assayed for Histone 3 Lysine 9 methylation.Importantly, alpha amanatin treatment inhibited siRNA EF52 mediatedHistone 3 Lysine 9 methylation (FIG. 11E) suggesting that RNA Pol-IImediated transcription is required for siRNA mediated TGS.

The initial discovery that promoter targeted siRNAs can induce genesilencing in human cells proved that small RNAs in mammals, Drosophila,C. elegans and plants can regulate gene expression by three conservedmechanisms: transcriptional gene silencing, mRNA degradation andtranslational inhibition. While there are many functional similaritiesbetween siRNA mediated TGS in mammals, Drosophila, C. elegans and plantsthe underlying mechanism may be somewhat varied. Data presented heresuggests that the de novo DNA methyltransferase enzymes of the DNMT3family are possibly guided by the small RNAs to the targeted promoter.The observation that DNMT-1 does not bind siRNA EF52 while DNMT3A and 3B(data not shown) do suggests that the de novo methyltransferases binddsRNA independent of the DNA binding domain (Datta et al., 2003; Xie etal., 1999). Interestingly, DNMT3A has been shown to bind siRNAs (Jefferyand Nakielny, 2004) as well as associate with histone deacetylase 1(HDAC1), the histone methyltransferase (Suv39H1), and HP1 (Fuks et al.,2003). Moreover, the observation that the antisense strand of EF52preferentially binds DNMT3A and co-immunoprecipitates in vivo withDNMT3A, H3K27, and the targeted promoter and is efficacious insuppressing HIV-1 Tat induced U3 mediated transcription suggests amechanism of action.

The emerging model for the mechanism of siRNA mediated TGS in humancells is proposed to operate temporally as: 1) the siRNA is eitherunwound and/or binds DNMT3A and then acted on by a helicase which then2) allows the antisense strand to direct the DNMT3a to the targetedpromoter leading ultimately to promoter site recognition. Next, 3) theDNMT3a/antisense siRNA complex may contain or then recruit HDAC-1 andSuv39H1 (Datta et al., 2003; Fuks et al., 2003; Fuks et al., 2001) whichcould 4) lead to the removal of the acetate and subsequent methylationof histone 3 lysine 9 and/or lysine 27 (Kawasaki and Taira, 2004) (FIG.13). The result of H3K9 and/or H3K27 methylation is the suppression ofthe particular targeted genes expression (Mutskov and Felsenfeld, 2004;Bachman et al., 2001). Finally, if the gene silencing is re-enforced andpositively selected for by the cell and it's local environment then DNAmethylation and permanent silencing of the antisense siRNA targeted genemay ensue. Indeed HDAC-1, DNMT3a/siRNA and the NuRD chromatin remodelingcomplex (Jeffery and Nakielny, 2004; Datta et al., 2003; Zhang et al.,1999) can all be linked indicating one potential pathway to siRNAmediated TGS. Interestingly, the observation that there is strandspecificity in the observed co-immunoprecipitations suggests two modelsfor siRNA/promoter recognition; 1) there is an antisense siRNA andnon-coding RNA interaction at the core of the targeting or 2) theunwinding of promoter DNA by RNA polymerase II allows for anantisense/DNA interaction to occur leading to promoter site recognitionand subsequent silencing.

Short interfering RNAs (siRNAs) have been shown to silence genes at thetranscriptional level in human cells (Morris et al., 2004b; Kawasaki andTaira, 2004; Kawasaki et al., 2005). Using human cells, we show thatEF1A promoter-directed siRNA EF52 binds DNMT3A and directs histonemethylation whereas controls do not. The binding of siRNA to DNMT3A wasspecific and showed a strand preference that co-immunoprecipitated withH3K27 and the targeted promoter. These results are the firstdemonstration that promoter directed siRNAs bind DNMT3A and co-localizeto the targeted promoter and as such suggest a mechanism for siRNAmediated TGS in human cells. Importantly, the observation that siRNAsdirect histone methylation and this effect is reversed by the inhibitionof RNA Pol-II suggests that transcription is required for siRNA mediatedTGS as well as that siRNAs may function to direct and/or write thehistone code. Taken together these data propose that siRNAs mediatecontrol of DNA in an RNA Pol-II mediated fashion through epigeneticmodifications specifically involving histone 3 methylation and DNMT3A.These findings propose that dsRNA, specifically the antisense strand,plays a pivotal and underappreciated role in regulating the cell thatcould be conceptualized to be used therapeutically in treating virtuallyany ailment affecting humans.

Example 13

Materials and Methods for Examples 14-18

Cell culture: We sequestered a reporter system that contains the CCR5promoter driving expression of a marker gene (red-shifted GFP). Thevector pR5-GFPsg143 contains ˜3 kb of CCR5 promoter, intron, and exons 1and 2 (Guignard et al., 1998; Moriuchi et al., 1997; Mummidi et al.,1997) and drives the expression of red-shifted GFP (a gift from Dr. G.N. Pavlakis) (Rosati et al., 2001). A total of 4.0×10⁶ 293T cells weretransfected with vector pR5-GFPsg143 (5 μg, Lipofectamine 2000™) andneomycin-selected (800Ag/ml) to generate the stable cell population(293T CCR5-GFP). HeLa stable cells expressing RASSF1A promoter-specificshRNAs or control vector alone were previously generated in our lab (agift from Dr. D. Castanotto) (Costanotto et al., 2005).

siRNA screening: To screen CCR5 promoter-specific siRNAs for knockdownof GFP expression, a total of 9.4×10⁵ 293T CCR5-GFP cells wereplated/well in a 12-well plate and 24 hrs later transfected with therespective promoter-specific siRNAs (Table 3) and the CCR5 mRNA controlsiRNA (10 nM) using MPG at a 10:1 charge ratio (MPG:siRNA), as describedin Morris et al. (2004b) and Morris et al. (1997). The respective siRNAswere constructed from oligonucleotides following previously establishedmethodologies for T7 expressed siRNA synthesis (Ambion Silencer™m). 48hrs post-transfection, cultures were collected for fluorescenceactivated cell sorting (FACS) analysis of GFP expression. TABLE 3 CCR5Specific siRNAs R5-25 5′-GCCAAAGCUUUUUAUUCUAaa-3′ (SEQ ID NO: 35)3′-aaCGGUUUCGAAAAAUAAGAU-5′ (SEQ ID NO: 36) R5-615′-GCCCAGAGGGCAUCUUGUGaa-3′ (SEQ ID NO: 37) 3′-aaCGGGUCUCCCGUAGAACAC-5′(SEQ ID NO: 38) R5-149 5′-CCGCCAAGAGAGCUUGAUAaa-3′ (SEQ ID NO: 39)3′-aaGGCGGUUCUCUCGAACUAU-5′ (SEQ ID NO: 40) R5-8545′-GCCCGUAAAUAAACUUUCAaa-3′ (SEQ ID NO: 41) 3′-aaCGGGCAUUUAUUUGAAAGU-5′(SEQ ID NO: 42) R5- 5′-AAUUCUUUGGCCUGAAUAAaa-3′ (SEQ ID NO: 43) Con-3′-aaUUAAGAAACCGGACUUAUU-5′ (SEQ ID NO: 44) trol

Chromatin immunoprecipitation: ChIP assays (Strahl-Bolsinger et al.,1997) were performed on 4.0×10 293T CCR5-GFP cells transfected with 30nM of synthetic (generated by IDT, Coralville, Iowa) R61 siRNA orcontrol R5 siRNA using Lipofectamine 2000w. Treated cultures wereformaldehyde cross-linked (1%, 10 min, room temp (RIT)) and then thereaction was stopped by adding glycine at a final concentration of 0.125M (10 min, R/T). The cells were then washed twice in 1×PBS+1/1000 PMSF(stock PMSF at 0.5M), resuspended in 600 μl of ChIP lysis buffer (50 mMHEPES pH 7.5, 140 mM NaCl, 10% Triton X100, 0.1% NaD, 1/1000 PMSF) andincubated on ice (10 min). Next, the samples were centrifuged (5,000rpm, 5 min, 4° C.), resuspended in 600 μl ChIP lysis buffer, incubatedon ice (10 min) and then sonicated (Branson 50 cell machine, 6 intervalswith 20 second pulses and 2 min rests). The sonicated samples were thencentrifuged (14,000 rpm, 10 min, 4° C.) and the supernatants removed andpre-cleared with 30 μl protein A/Salmon Sperm (Upstate, Charlottesville,Va., catalog #16-157) (15 min, 4° C., rotating platform). Thepre-cleared supernatants were then centrifuged (14,000 rpm, 5 min, 4°C.) and supernatants removed and divided into equivalent aliquots. Thepartitioned samples were incubated with no antibody (control),anti-H3K9^(me2+) (Upstate catalog #07-441), anti-Ago1 (Upstate catalog#07-599), anti-RNAPII (Abcam, Cambridge, Mass., catalog # ab817),anti-H3K27^(me3+) (Upstate catalog #07-449), anti-TRBP antiserum (a giftfrom Dr. A. Gatignol) (Duarte et al., 2000), and anti-Ago2 (Upstatecatalog #07-590) (3 hrs to overnight, 4° C., rotating platform). Thesamples were then treated with 10 μl Protein A/Salmon Sperm (Upstate),(15 min, R/T, rotating platform), pulled-down (10,000 rpm, 1 min, 4°C.), and washed. The no antibody control supernatants were saved andused as input controls. The washes consisted of 2 washes with 1 ml ofChIP lysis buffer, 2 washes with 1 ml ChIP lysis buffer high salt (50 mMHEPES pH 7.5, 500 mM NaCl, 1% Triton X100, 0.1% NaD, 1/1000 PMSF),followed by 2 washes with 1 ml ChIP wash buffer (10 mM Tris pH 8.0, 250mM LiCl, 0.5% NP-40, 0.5% NaD, 1 mM EDTA). For each wash the sampleswere incubated (3 min, R/T, rotating platform), followed bycentrifugation (14,000 rpm, 3 min, R/T). After the final wash thecomplexes were eluted by two treatments of 100 μl elution buffer (50 mMTris pH 8.0, 1% SDS, 10 mM EDTA)(10 min, 65° C.), followed bycentrifugation (14,000 rpm, 3 min, R/T). The eluted complexes along withthe initial aliquot used in the no antibody control (200 μl) were thenreverse cross-linked by adding 1 μl RNase A (10 mg/ml) and 20 μl of 5MNaCl to each sample and incubated (4-6 hrs, 65° C.). The reversecross-linked samples were then treated (10 μl of 0.5M EDTA, 20 μl of 1MTris-HCl pH 6.5, 2 μl of 10 mg/ml Proteinase K)(1 hr, 45° C.) and theDNA recovered by Phenol/Chloroform extraction and assayed usingreal-time PCR (40 cycles of 94:55:72° C. at 30:30:30 seconds) withprimers 5′chip-2 5′-GGG GTC TCA TTT GCC TTC TTA GAG ATC ACA-3′ (SEQ IDNO:45) and 3′chip-3 5′-TAA GTA TAT GGT CAA GTT CAG GTT C-3′ (SEQ IDNO:46) that specifically overlap the CCR5 promoter R61 siRNA targetsite, standardized to plasmid pR5-GFPsg143, and normalized to inputvalues. To determine the extent of Ago1 and H3K9^(me2+) spreading,primers 5′walk-15′-GTC TTC TCA GCT CTG CTG ACA ATA CT-3′ (SEQ ID NO:47)and 3′walk-25′-GGA TTT TCA CTC TGT TCA CTA TTT TGT TGC-3′ (SEQ ID NO:48)that overlap a region ˜100 to 300 bp downstream of the CCR5 promoter R61siRNA target site were used. For RASSF1A promoter ChIP experiments,primers 5′ras-1 5′-GAA GGA AGG GCA AGG CGG GGG GGG CTC TGC-3′ (SEQ IDNO:49) and 3′ras-1 5′-GGC CCG GTT GGG CCC GTG CTT CGC T-3′ (SEQ IDNO:50) were used.

qRT-PCR amplification: SuperScript™ III Platinum SYBR Green One-StepqRT-PCR kit (Invitrogen, Carlsbad, Calif.) was used to amplify GFP,RASSF1A, and GAPDH transcript levels from total RNA isolated with RNASTAT-60 ™ (Tel-Test, Friendswood, Tex.), using GFP and GAPDH primers aspreviously described (Morris et al., 2004b). RASSF1A primers used wereURFIA 5′-TGG TGC GAC CTC TGT GGC GAC TT-3′ (SEQ ID NO:51) and RT45′-GATGAA GCC TGT GTA AGA ACC GTC CT-3′ (SEQ ID NO:52) as previously described(Costanotto et al., 2005).

Western analysis and RNase treatment: Total protein from 293T CCR5-GFPwhole cell extracts, anti-RNAPII (Abcam, Cambridge, Mass., catalog #ab817) immunoprecipitates, anti-RNAPII immunoprecipitates from cellextracts treated for 30 min in 50 μg /ml RNase A (Sigma, St. Louis, Mo.)at 25° C., and extracts from Ago1 siRNA treated [Ago1(−)] or control R5siRNA treated [Ago1(+)] 293T CCR5-GFP cells were heated (5 min, 95° C.),separated by electrophoresis in 4-12% SDS polyacrylamideelectrophoresis, transferred to PVDF membranes, probed with anti-Ago1(Upstate catalog # 07-599), and developed with anti-rabbit horseradishperoxidase-labelled antibodies (Amersham Biosciences, Pittsburgh, Pa.)and Luminol detection reagent (Fisher, Hampton, N.H.).

Promoter methylation analysis: Genomic DNA was digested for 1 hour withAva I (New England Biolabs, Ipswich, Mass.) or Apa I (New EnglandBiolabs) at 37° C. for R61 or R5 control siRNA treated 293T CCR5-GFPcells or RASSF1A shRNA or control vector expressing HeLa stable cells,respectively and used as templates for promoter specific real-time PCR(40 cycles of 94:55:72° C. at 30:30:30 seconds) with ChIP primers forthe CCR5 and RASSF1A promoters. PCR amplification indicates that the AvaI or Apa I sites within the targeted promoter sequences are methylatedand as such protected from enzyme digestion. All values were normalizedto equivalent amounts of undigested genomic DNA samples incubated inNEBuffer #4 alone.

Example 14

siRNA Mediated Silencing of CCR5 Promoter

To measure the levels of siRNA mediated silencing at the targeted CCR5promoter, we generated a stable cell line expressing CCR5promoter-driven green fluorescent protein (293T CCR5-GFP). Fourcandidate siRNAs with sequence homology to the CCR5-GFP promoter (Table3) were screened for inhibition of GFP expression at 48 hrs post-siRNAtransfection, with two siRNAs (R61 and R149) showing ˜50% reduction ofprotein levels (FIG. 14). GFP mRNA levels were measured at 24 hrspost-siRNA transfection using real-time quantitative RT-PCR (qRT-PCR)and normalized to GAPDH levels. In cells treated with promoter-specificR61 siRNA, we observed ˜69% knockdown of GFP mRNA transcript levels whencompared to R5 control siRNA (CCR5 mRNA-specific) transfected cells(FIG. 15A), similar to previous observations with siRNAs targeted toRNAPII promoters (Morris et al., 2004b; Weinberg et al., 2006; Ting etal., 2005). Furthermore, we examined siRNA mediated TGS at theendogenous RASSF1A promoter using HeLa cell lines stably expressing ashort hairpin RNA (shRNA) targeted to the RASSF1A promoter or a controlvector not expressing an shRNA (Castanotto et al., 2005). RASSF1A mRNAtranscript levels exhibited ˜74% knockdown by promoter-targeted shRNAsin this constitutively expressed setting (FIG. 15A).

Example 15

siRNA Mediated Induction of Silent Histone Modifications

Previous work has shown that siRNA mediated TGS correlates with silenthistone methylation marks, and H3K9^(me2+) and H3K27^(me3+) have beenfound to associate with siRNA targeted promoters (Weinberg et al., 2006;Ting et al., 2005). To determine whether the CCR5 promoter-specific R61siRNA could induce silent histone modifications, we screened theCCR5-GFP promoter specifically at regions overlapping the R61 siRNAtarget site using chromatin immunoprecipitation (ChIP) for H3K9^(me2+).A ˜14-fold enrichment of H3K9^(me2+) was observed at 24 hrs post-R61siRNA transfection, relative to R5 control siRNA transfected cells (FIG.15B) and consistent with previously observed epigenetic modifications(Weinberg et al., 2006; Ting et al., 2005). To test whether spreading ofH3K9^(me2+) to adjacent nucleosomes was occurring, we performed ChIPexperiments using PCR primers spanning a region ˜100 to 300 bpdownstream of the R61 siRNA target site. A ˜7-fold enrichment ofH3K9^(me2+) was observed downstream of the promoter target site (FIG.15B). Time-course ChIPs of the CCR5-GFP promoter showed an increase inH3K9^(me2+) at the targeted promoter between 12 and 24 hrs post-siRNAtransfection (FIG. 15C). However, only a negligible amount of DNAmethylation was observed at the targeted CCR5-GFP promoter, while aslight increase in DNA methylation occurred when constitutivelyexpressed shRNAs were targeted to the RASSF1A promoter (FIG. 16).

Example 16

Contribution of Ago1 or Ago2 to siRNA Mediated Promoter Silencing

We next investigated whether Ago1 or Ago2 might contribute to siRNAmediated promoter silencing, as Ago1 directs the induction and spreadingof H3K9^(me2+) and TGS in S. Pombe (Verdel et al., 2004; Noma et al.,2004), and Ago2 is a component of the well-characterized RNA-inducedsilencing complex (RISC) in human cells (Liu et al., 2004). ChIPexperiments in 293T CCR5-GFP cells transfected with R61 or R5 controlsiRNAs showed an ˜18-fold enrichment of Ago1 at the CCR5-GFP promoter inR61 siRNA transfected cells (FIG. 17A). Interestingly, a ˜14-foldenrichment of Ago1 downstream of the R61 target site was also observed(FIG. 17A), suggesting that spreading of Ago1 may direct and/orassociate with downstream histone modifications, leading to thespreading of TGS along the targeted gene. Additionally, Ago1 was alsoenriched at the shRNA-targeted endogenous RASSF1A promoter (FIG. 17A).We did not, however, observe enrichment of Ago2 in our ChIP experimentsin 293T CCR5-GFP cells transfected with R61 or R5 control siRNAs (FIG.18). To determine the temporal nature of Ago1 association with siRNAtargeted promoters, time-course ChIPs were conducted at 6 hr intervalsfrom 12 to 30 hrs post-siRNA transfection in 293T CCR5-GFP cells. Thetime-course ChIP experiments demonstrated a transient associationbetween Ago1 and the targeted promoter (FIG. 17B), correlating with aconcomitant increase in H3K9^(me2+) (FIG. 15C). These data suggest thatAgo1 directs siRNA mediated TGS of RNAPII promoters and acts upstream ofthe histone modification pathway.

Recent reports in S. Pombe (Kato et al., 2005) and in human cells(Weinberg et al., 2006) have also demonstrated that RNAPII is requiredfor siRNA mediated TGS. To determine whether RNAPII associates withAgo1, we performed co-immunoprecipitations from 293T CCR5-GFP cellextracts and found that Ago1 co-immunoprecipitated with RNAPII (FIG.17C). To test whether this association between Ago1 and RNAPII was via asingle-stranded RNA intermediate that is transcribed through promoterregions, 293T CCR5-GFP cell extracts were treated with RNase A. RNase Atreatment had no effect on the ability of RNAPII to co-immunoprecipitateAgo1 (FIG. 17C), suggesting a direct protein-protein interaction betweenan Ago1-containing transcriptional silencing complex and RNAPII.

Example 17

RNAi Mediated Knockdown of Ago1

RNAi mediated knockdown of Ago1 was next used to investigate therequirement of Ago1 in directing di-methylation of H3K9 and silencinggene expression. 293T CCR5-GFP cells were transfected with a validated,Ago1 mRNA-specific siRNA (Meister et al., 2004), and knockdown of Ago1expression was determined at 48 hrs post-Ago1 siRNA transfection (FIG.19A). Ago1 siRNA treated cells [Ago1(−)] or R5 control siRNA treatedcells [Ago1(+)] were transfected with promoter-specific R61 siRNAs at 24hrs following the Ago1 siRNA or R5 control siRNA transfections. GFPtranscript levels were markedly elevated in Ago1(−) cells at 24 hrspost-R61 siRNA transfection, relative to R61-transfected Ago1(+) cellswhich exhibited typical levels of Ago1 expression (FIG. 19B). Knockdownof Ago1 resulted in the loss of Ago1 binding at the targeted CCR5-GFPpromoter in ChIP experiments, which also correlated with a noticeablereduction in the levels of H3K9 (FIG. 19C). These data indicate thatAgo1 localization to the targeted promoter region is required for H3K9di-methylation and siRNA mediated TGS in human cells.

Example 18

ChIP Analysis of Endogenous CCR5 Promoter

Evluation of the endogenous CCR5 promoter in HEK 293 and HeLa cellsthrough ChIP experiments revealed that Ago1 also localized to theepigenetically silenced CCR5 promoter in both cell types (FIGS. 20A and20B). Ago2 was not observed in HEK 293 cells in our ChIP experiments ofthe endogenous CCR5 promoter (FIG. 18), analogous to our Ago2 ChIP dataat the siRNA targeted CCR5-GFP promoter in 293T CCR5-GFP cells.Supporting our observation that RNAPII co-immunoprecipitates with Ago1at epigenetically silenced promoters, RNAPII was also present at thesilenced CCR5 promoters in both HEK 293 and HeLa cells, as determined byChIP (FIGS. 20A and 20B). These data suggest that low levels of RNAPIItranscription of endogenously silenced promoters are required tomaintain an epigenetically silent state. Enrichment of H3K27^(me3+) wasobserved at the endogenous CCR5 promoters in both cell types (FIGS. 20Aand 20B), indicating the presence of a histone mark that is known torecruit the PcG repressor proteins to regions of facultativeheterochromatin.

A recently characterized component of the RNAi machinery is the HIV-1TAR RNA-binding protein 2 (TRBP2), a double-stranded RNA-binding proteinthat has been shown to be a component of the effector complex RISC(Forstemann et al., 2005; Gatignol et al., 2005; Gregory et al., 2005;Haase et al., 2005; Lee et al., 2006). We sought to determine whetherTRBP2 might also be associated with Ago1 in a nuclear transcriptionalsilencing complex. We utilized anti-TRBP2 antiserum (kindly supplied byA. Gatignol) to perform ChIPs of the CCR5 and RASSF1A promoters in HeLacells. TRBP2 was enriched at the endogenous CCR5 promoter in HeLa cellsat levels similar to Ago1 enrichment (˜5.23 and 18 5.59-fold enrichment,respectively) (FIGS. 20B and 20C), suggesting a nuclear transcriptionalsilencing complex composed of Ago1 and TRBP2. Furthermore, TRBP2localized to the shRNA-targeted and Ago1 enriched RASSF1A promoter (FIG.20C). These findings suggest an endogenous mechanism of transcriptionalregulation involving several components of the RNAi machinery, RNAPIItranscription, and Polycomb group proteins, all of which may act inconcert to mediate formation and maintenance of facultativeheterochromatin.

The current paradigm for the mechanism of TGS in human cells involvesH3K9, H3K27, and DNA methylation at the siRNA targeted promoters (Morriset al., 2004b; Castanotto et al., 2005; Buhler et al., 2005; Janowski etal., 2005; Zhang et al., 2005; Suzuki et al., 2005), although therequirement of DNA methylation for TGS in human cells is still uncertain(Ting et al., 2005; Janowski et al., 2005; Park et al., 2004; Svoboda etal., 2004) and may possibly be promoter-dependent. Data presented herereveal that Ago1 directs siRNA mediated TGS by associating with targetedpromoters through an interaction with RNAPII. The finding that Ago1 isrequired for siRNA mediated promoter silencing and H3K9^(me2+), coupledwith the observation that transient association of Ago1 at the targetedpromoter corresponds with an increase in H3K9^(me2+), suggests that Ago1functions upstream of chromatin modifications that silence geneexpression by recruiting specific histone methyltransferases such as G9a(H3K9^(me2+)) and/or EZH2 (H3K27^(me3+)) (Vire et al., 2006).

Along with previously published observations (Morris et al., 2004b;Weinberg et al., 2006; Ting et al., 200; Buhler et al., 2005; Suzuki etal., 2005), the findings presented here suggest a putative model forsiRNA mediated TGS in human cells involving a transcriptional silencingcomplex (TSC) containing Ago1, TRBP2, siRNA, and possibly chromatinremodeling factors (i.e. HDAC-1, G9a, EZH2, DNMT3a) (Weinberg et al.,2006; Morris et al., 2005) (FIG. 21). The TSC may be directed by siRNAsto their target promoters in an RNAPII-dependent manner (Weinberg etal., 2005), and the observation here that Ago1 associates with RNAPIIsuggests that RNAPII may provide a docking site for the TSC. Upon siRNAloading into the TSC, the antisense strand (Weinberg et al., 2006) mayguide the TSC to a low copy promoter-specific RNA (PRNA) thatcorresponds to the siRNA targeted promoter (manuscript in preparation:Han, Kim, Rossi, Morris). This would allow for the formation of anRNA:RNA duplex between the antisense strand of the siRNA and either anascent pRNA while it is being transcribed or a pRNA that is already acomponent of the local chromatin structure (Maison et al., 2002).Recognition of the siRNA target site would potentially stall thepRNA-scanning TSC:RNAPII complex and initiate the formation offacultative heterochromatin by recruiting histone methyltransferases andpossibly PcG repressor complexes, which have recently been linked toAgo1 and the RNAi machinery in Drosophila (Grimaud et al., 2006). Theinclusion of TRBP2 in the TSC suggests a potentially important role forthis protein in Ago1 mediated RNA binding.

An alternative model implicated by the observed spreading of TGS andfacultative heterochromatin from a promoter nucleation site wouldinvolve the siRNA antisense strand-directed TSC:RNAPII complex movingalong the targeted RNAPII-transcribed promoter/gene, potentiallymodifying the H3 histones as they are reconstituted into nucleosomesimmediately following transcription. Both of these models, or anamalgamation of the two, would necessitate the involvement of RNAPII,which is consistent with recent evidence that RNAPII function isrequired for histone methylation and TGS at siRNA-targeted promoters inhuman cells (Weinberg et al., 2006) and in S. Pombe (Kato et al..,2005), suggesting an Ago1 and RNAPII-dependent mechanism oftranscriptional silencing that is evolutionarily conserved.Additionally, the recent discovery and characterization of a vast arrayof small (21- to 26-nt), non-coding RNAs is changing the classicalunderstanding of gene regulation (Katayama et al., 2005), and takentogether with the data presented here, suggests that these non-codingRNAs may play a more profound role in writing the histone code (Jenuweinand Allis, 2001) and regulating gene expression at the level of DNA.

Example 19

Additional Data Supporting Ago1 Involvement

We have generated new data that further supports the connection betweenArgonaute 1 directed transcriptional gene silencing (TGS) and Polycombgroup mediated epigenetic silencing in human cells. We have performedchromatin immunoprecipitations in HeLa cells with a recently acquiredantibody (Upstate) against the H3K27^(me3+) histone methyltransferaseand Polycomb group protein EZH2. Our CHIP data shows enrichment of EZH2at the shRNA-targeted RASSF1A promoter (FIG. 22), suggesting that anAgo1 containing transcriptional silencing complex (TSC) may recruit EZH2to epigenetically silence the targeted RASSF1A promoter. Furthermore,EZH2 was also found to be enriched at the endogenously silenced CCR5promoter (FIG. 22), which we previously demonstrated was also enrichedfor Ago1 (FIG. 20B), suggesting that Ago1 is involved in the mechanismof endogenous epigenetic silencing at regions of facultativeheterochromatin. Recent genome-wide mapping for Polycomb components inhuman cells has also shown that CCR5 is a Polycomb target gene (Brackenet al., 2006).

Additionally, the low levels of DNA methylation that we have observed atthe shRNA-targeted RASSF1A promoter (FIG. 14; Castanotto et al. 2005)may result from the recruitment of DNMT3a, shown to associate withpromoter-targeting siRNA (Weinberg et al., 2006), by EZH2, which hasrecently been shown to recruit DNMT3a (Vire et al., 2005).

We also performed ChIPs in HeLa cells of the human Polycomb targetpromoter MYT1 (Kirmizis et al., 2004) and found enrichment of Ago1,EZH2, and H3K27^(me3+) at this epigenetically silenced promoter (FIG.23). Collectively, these data provide additional evidence in support ofthe connection between RNAi and Polycomb silencing in human cells.

The use of the terms “a” and “an” and “the” and similar referents in thecontext of describing the invention (especially in the context of thefollowing claims) are to be construed to cover both the singular and theplural, unless otherwise indicated herein or clearly contradicted bycontext. The terms “comprising,” “having,” “including,” and “containing”are to be construed as open-ended terms (i.e., meaning “including, butnot limited to,”) unless otherwise noted. Recitation of ranges of valuesherein are merely intended to serve as a shorthand method of referringindividually to each separate value falling within the range, unlessotherwise indicated herein, and each separate value is incorporated intothe specification as if it were individually recited herein. All methodsdescribed herein can be performed in any suitable order unless otherwiseindicated herein or otherwise clearly contradicted by context. The useof any and all examples, or exemplary language (e.g., “such as”)provided herein, is intended merely to better illuminate the inventionand does not pose a limitation on the scope of the invention unlessotherwise claimed. No language in the specification should be construedas indicating any non-claimed element as essential to the practice ofthe invention.

It will be appreciated that the methods and compositions of the instantinvention can be incorporated in the form of a variety of embodiments,only a few of which are disclosed herein. Embodiments of this inventionare described herein, including the best mode known to the inventors forcarrying out the invention. Variations of those embodiments may becomeapparent to those of ordinary skill in the art upon reading theforegoing description. The inventors expect skilled artisans to employsuch variations as appropriate, and the inventors intend for theinvention to be practiced otherwise than as specifically describedherein. Accordingly, this invention includes all modifications andequivalents of the subject matter recited in the claims appended heretoas permitted by applicable law. Moreover, any combination of theabove-described elements in all possible variations thereof isencompassed by the invention unless otherwise indicated herein orotherwise clearly contradicted by context.

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1. A method of reducing gene expression of a target gene in a cellcomprising exposing said cell to an siRNA molecule which is specific fora target sequence in the target gene, wherein the siRNA molecule causesreduced gene expression.
 2. The method of claim 1, wherein the siRNAmolecule is directed to the promoter region of the gene.
 3. The methodof claim 1, wherein the siRNA molecule binds to a sequence within about150 bp of the transcription start site.
 4. The method of claim 1,wherein the antisense strand of the siRNA molecule is specific for thetarget sequence.
 5. The method of claim 1, wherein the siRNA moleculeincreases methylation of histones associated with the target gene. 6.The method of claim 5, wherein the methylation of histones is mediatedby Ago1.
 7. The method of claim 1, wherein the cell is a mammalian cell.8. The method of claim 7, wherein the mammalian cell is a human cell. 9.The method of claim 1, wherein said siRNA contains about 18-29 basepairs.
 10. The method of claims 9, wherein said siRNA contains about18-21 base pairs.
 11. The method of claims 9, wherein said siRNAcontains about 18-23 base pairs.
 12. The method of claims 9, whereinsaid siRNA contains about 19-23 base pairs.
 13. The method of claims 9,wherein said siRNA contains about 24-28 base pairs.
 14. The method ofclaims 9, wherein said siRNA contains about 21-26 base pairs.
 15. Themethod of claim 1, wherein the cell is exposed to the siRNA byintroducing into the cell DNA sequences encoding a sense strand and aantisense strand of the siRNA, wherein the siRNA is expressed in thecell.
 16. The method of claim 15, wherein the introducing isaccomplished using at least one vector.
 17. The method of claim 16,wherein the vector is a plasmid vector.
 18. The method of claim 16,wherein the vector is a viral vector.
 19. The method of claim 18,wherein the viral vector is a retroviral vector, a lentiviral vector, oran adenoviral vector.
 20. The method of claim 16, wherein the vector isan adeno-associated vector.
 21. The method of claim 16, wherein theintroducing takes place in vivo.
 22. The method of claim 16, wherein theintroducing takes place in vitro.
 23. The method of claim 16, whereinthe introducing is achieved via transformation, transduction,transfection, or infection.
 24. The method of claim 16, wherein saidintroducing is achieved via a liposome.
 25. The method of claim 16,wherein said introducing is achieved via a liposome.